A good rule of thumb is to allow yourself two minutes for each 1,000 feet of altitude you need to lose. So, if you're cruising at 10,000 feet above field elevation, start descending 20 minutes before your planned arrival.

What is the 3 6 rule?

For larger aircraft, typically people use some form of the 3/6 Rule: 3 times the altitude (in thousands of feet) you have to lose is the distance back to start the descent; 6 times your groundspeed is your descent rate.

What speed should you descend at?

A rule of thumb is to multiply ground speed by 5 to get the required rate of descent in feet per minute e.g. an airplane with a ground speed of 300 knots must descend at (300 x 5) 1500 feet per minute to maintain a 3o profile.