Here are two options to consider.
DISTANCE
When cruising at 10,000 feet. (10,000/1,000) x5 = 50 miles out and this allows for most piston airplanes to maintain 500 ft/min all the way down to desired altitude.
Commercial and Jet aircraft use a multiplier of 3 and 2000 ft/min.
Many pilots use what they call the 3-to-1 rule (three miles distance per thousand feet in altitude.) This is a good rule for determining when to start your descent.
Take your altitude in feet, drop the last three zeros, and multiply by 3. So if you are at 12000 ft. agl, means 12 multiply by 3 equals 36 miles out to start your descent. You can check your DME from airport if it you have one.
TIME
Another 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.
So whether you use time or distance, is up to you on how comfortable you feel.
