When to start descent is probably one of the most asked questions I come across on a day to day. A comfortable descent rate is 3 degrees, but when do you start your descent?
One way to calculate that is by dividing the altitude you need to lose by 300. For example, if you’re at 11,000ft, and you need to get down to a pattern altitude of 2,000ft, you need to descend 9,000ft.
9,000/300 = 30 miles.
If you start a 3-degree descent 30 miles out, you’ll hit pattern altitude as you reach the airport.
This doesn’t apply for all aircraft but for most you may fly it should do the job.