How far away from the airport is the descent for approach and landing initiated?
As J. Hougaard said, 3 nautical miles per 1000 feet is the general rule. However, large aircraft that can vary greatly in weight may vary that figure. Also, best to factor in whether tailwinds or headwinds in the descent.

In 747-100/200 freighters in the 1990s, we used the 3 nautical miles per 1000 feet most of the time. However, if we were heavy, i.e. we would be landing right up against the max landing weight, we would use 3.5, and especially if we would be descending in a tailwind. If we were light, in other words no cargo, 2.5 was a good figure to use. And, of course, you didn't have to use exactly 3.5, 3.0 or 2.5. What I often did was figure the 3.0 distance and then just add or subtract 10 miles or so.

In one memorable instance, I used 2.0 and it worked, but just barely. I had heard that if you had an empty aircraft and used a maximum drag procedure it could be done. I only had a chance to try it once going into Hong Kong's old Kai Tak airport on a clear day with no traffic, straight in to rwy 31.