For those of you who are unsure how far away from destination you should commence your descent, here is a handy little rule of thumb that we use in real world flying:
Height to lose x 3 = distance required
Cruising at 11,000 feet AMSL. Landing airfield elevation is 700 feet (rounded up 1000 feet for easy maths).
11,000 feet - 1,000 feet = 10,000
000x 3 = 30
Therefore, top of descent point is 30 nautical miles from your destination
Cruising at 25,000 feet AMSL. Landing airfield elevation is 0 feet (sea level).
25,000 feet - 0 feet = 25,000
000x 3 = 75
Therefore, top of descent point is 75 nautical miles from your destination airfield.
This formula works well for flying most conventional type fixed wing aircraft.
However, for faster aircraft, you may need to factor in a level segment for slowing the aircraft down after descending at a high speed so that you can configure for landing with flap and gear.
This is typically done prior to the Initial Approach Fix (IAF), which is often found at a distance of 10 miles from the landing runway at a height of 3000 feet AGL.
Use the same formula, and simply add 5nm for your level segment on top of the distance calculated for your descent.
Cruising at 25,000 feet AMSL. Landing airfield elevation is 0 feet (sea level). The IAF is at 3000 feet AMSL at a distance of 10nm.
25,000 feet - 3000 feet = 22,000
000x 3 = 66
66 + 5 mile level segment = 71
Therefore, top of descent point is 71 nautical miles from your IAF.
Working back from the runway which in this case is 10nm from the IAF, 71 + 10 = a top of descent point 81 nautical miles from the runway.