What does it take to turn a 747 around 180 degrees?
How long, and how wide of a turn depends entirely on three things:


What is the true airspeed.
What is the bank angle of the turn?
Is level flight maintained during the turn.


Since it's easiest to calculate, and presumably what you intended, we'll assume #3 is true (level flight). However, if you really needed to get turned around fast, turning in a descent will give you a tight turn without the extra g-forces.

Cruise Speed for a 747 depends on altitude, generation, company guidelines, etc, but we'll go with Mach 0.85 for this example, which seems to be in the ballpark. If we say it's cruising at 35,000 feet in standard atmospheric conditions, that comes out to 490 knots TAS.

Next, pick a bank angle. A reasonable bank angle for that airplane with passengers on board is 25°, and you could probably get away with 30°. Anything beyond that will cause passengers to complain. Of course, if it's an emergency you might consider something higher. Just remember, the higher the bank angle, the stronger the g-forces which will be felt by the people inside, and the aircraft itself, which has structural limitations. The G-force can be calculated using:

$$
gforce = \frac{1}{\cos(bank)}
$$



For an example, let's select 25° bank since that's the most realistic.

Radius

Calculate the radius of the turn using this formula, slightly modified from Wikipedia to give nmi instead of feet:

$$
Radius\ of\ turn\ in\ nautical\ miles= \frac{velocity^2}{68579 \times \tan(bank)}
$$

Which gives us:

$$
7.51nmi = \frac{490^2}{68579 \times \tan(25°)}
$$

So the turn itself would be about 15 nautical miles wide (≈ 91,000 feet) not accounting for wind.

Distance Traveled

Using basic geometry, the distance traveled for a 180° turn is $d = r\pi$ (half the circumference of a circle), so:

$$
23.59= 7.51\pi
$$

Giving us 23.59 nautical miles traveled.

Duration

$$
time\ in\ minutes = \frac{distance \times 60}{TAS}
$$

Therefore, at 490 knots (nautical miles per hour) and 25° bank, it would take about 2 minutes, 53 seconds to complete the turn.

$$
2.89 = \frac{23.59 \times 60}{490}
$$

You could go through and calculate the results for any speed and bank angle you want. This is true regardless of the type of airplane in question.



Turning in a Descent

Your edited question asks how much altitude would be lost if it were not a level turn. There is also no single answer to this question because it depends entirely on how you perform the maneuver.

One reason you may have to descend in a turn is because the increased lift results un increased induced drag, and the aircraft engines may not have enough power to compensate, resulting in the decreased airspeed, and possibly a stall. In this case, the equations for calculating the turn radius are exactly the same as in level flight. The amount of altitude lost will depend on the descent rate required in order maintain airspeed, which will vary depending on the engine power available, the weight of the aircraft, and the drag curve at the given speed and AoA. Peter Kämpf's answer gives an example of what this might look like for a 747 in a 1.5g turn.

Another version of a turning descent would be an accelerated descent. The advantage in this case it that it would allow turns to be made without additional g-force. The disadvantage is that you'll be accelerating downwards instead of descending at a constant rate. This can get out of hand very quickly and would only be good for very short duration turns.

To illustrate this disastrous option, let's look at what would happen in a 1g turn at the same 25° and 490 knots TAS. Since the math is more complicated, it'll be easier to talk about this portion in metric units. Here's a conversion table:

1 nautical mile = 1852 meters
1 knot = 0.514444 meters per second
1 foot = 0.3048 meters


First, in order to maintain a constant 1g, the lift vector is simply rotated in the turn (instead of rotated and increased in order to maintain altitude). Therefore, the magnitude of our lift vector will be equal to the acceleration due to gravity near the Earth's surface. We'll round that to 9.8 m/s/s.

Inward Acceleration

Our acceleration towards the center of the turn $a_c$ (the portion of our lift vector pointed inwards instead of up) can be determined using the following equation where $\theta$ is the bank angle:

$$
\sin(\theta) = \frac{a_c}{g}
$$

Solving for $a_c$:

$$
a_c = g\sin(\theta)
$$

Therefore

$$
4.142 = 9.8\sin(25°)
$$

Radius

So 4.142 meters per second per second is how fast, at 25° of bank, and 1g, we'll be accelerating towards the center of the turn. With that, information, along with our known velocity, we can calculate the radius of the turn using this equation where $v$ is our velocity in meters per second, and $R$ is the radius in meters:

$$
R = \frac{v^2}{a_c}
$$

Plug in our numbers:

$$
15341 = \frac{(490 \times 0.514444)^2}{4.142}
$$

This comes out to a radius of 15341 meters (8.28 nautical miles).

Distance Traveled

Now that we have our radius, we can calculate how long it will take us to turn 180°. This part is the same equation as before.

$$
48195 = 15341\pi
$$

Giving us 48195 meters (26 nautical miles).

Duration

Distance divided by velocity gives us duration.

$$
t = \frac{d}{v}
$$

In our case:

$$
191.2\ seconds = \frac{48195}{490 \times 0.514444}
$$

So, it'll take 3 minutes 11 seconds to complete the turn.

Downward Acceleration

The last value we need before we can calculate the altitude lost in the turn is to calculate how fast we're going to be accelerating towards the ground in this turn. Lets first calculate the upward portion of our lift vector:

$$
a_u = g\cos(\theta)
$$

Therefore

$$
8.882 = 9.8\cos(25°)
$$

Our lift vector is accelerating us upwards at 8.882 m/s/s, while gravity is trying to pull us down at -9.8 m/s/s. That comes out to a net vector of -0.918 m/s/s.

Altitude Lost

This part requires a little bit of calculus because our descent rate is accelerating. The integral of acceleration is velocity ($v = at$), and the integral of velocity is distance:

$$
d = \frac{at^2}{2}
$$

So, lets calculate the change in vertical distance (altitude):

$$
-16780 = \frac{-0.918 \times 191.2^2}{2}
$$

So, theoretically, our airplane lost 16,780 meters of altitude (55,052 feet). Of course, since we were only 35,000 feet in the air to begin with, this is rather bad news for us. Besides hitting the ground, you would also come close to risking structural damage to the airframe because in this example, 490 knots is considered the speed over the ground, but the total velocity would be higher (about 596 knots) by the end of the turn due to the sink rate.

You'll also notice that, assuming we had the altitude to lose in the first place, the turn took longer to complete than in level flight. This is because the magnitude of the lift vector was smaller.

You can feel free to experiment with other speeds and bank angles, and you could also experiment with higher g turns (simply replace $g$ in the equations with $2g$ or similar). In some cases higher than 1g, you may actually gain altitude, though it's unlikely a 747 would be able to sustain a high-g maneuver which gains altitude.

As a quick second example, consider a 2g turn with 80° of bank at 400 knots:

$$
a_c = 19.302 = 2g\sin(80°) \\
R = 2193.8 = \frac{400 \times 0.514444}{a_c} \\
d = 6892 = R\pi \\
t = 33.5 seconds \\
a_u = 3.404 = 2g\cos(80°)
$$

Giving a total altitude lost of 3589 meters (11,775 feet) over 33.5 seconds. However, at the end of the turn, you would have a sink rate of 214 meters (703 feet) per second. That's over 42,000 feet per minute. It would likely be possible to recover from that in the altitude remaining, but it would not be pleasant.