How long after takeoff for a Boeing 747-400 to reach cruise speed?
I did a search on FlightAware for 747-400 aircraft in flight, and sorted by the estimated time enroute. A Qantas flight from KLAX to YSSY came up. I'm assuming that for a 6500 Nm flight, at 13.5 hours, the aircraft is fairly heavy at takeoff.

The flight data shows a departure at 03:28:33, reaching an initial cruise level of FL300 at 03:52:59, after a continuous climb. It was at cruise speed at this point, which gives a time of 25 minutes after takeoff. Comparing the coordinates of these two reporting points gives a distance of about 180 Nm, which from the track is pretty straight.

If you just want to know when it reached a speed around that of cruise, this was about 10 minutes and 80 Nm earlier in the climb, when it reached a ground speed of about 480 knots.