I am wondering if there is a rule-of-thumb formula for the speed, altitude and the distance from the airport that the descent for approach to land is initiated? I see airlines do it about 25 to 30 minutes before landing. When, say a King Air, TBM, etc.,...doing 275 Kts at 28,000 feet will start the decent for landing at a sea-level airport, barring unusual weather or traffic conditions?
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.
The widely used rule-of-thumb says that you need 3 nautical miles (NM) to lose 1000 feet of altitude. So an airliner cruising at 36.000 ft will begin its descend around 110 NM before touchdown, whereas a smaller turboprop airplane crusing at say 18.000 ft will begin its descend about 55 NM before touchdown.
The speed used during descend and approach varies greately as it depends on the type of aircraft.