Any approximate method is going to vary for different types of plane, depending on their speed, altitude and descent rate, but here's how I figure it for my Cessna 172.
I know I'm traveling around 2NM a minute (actually a little slower), and I like to descend around 500 fpm.
So if I have to drop 5,000 feet to pattern altitude, the TOD is 10 minutes out (5,000 feet / 500 fpm). At 2NM a minute, that is 20 miles out.
So if I have my GPS / DME set to my destination airport, I am looking for both ETE: 10 min, and also Dist: 20 miles, then I start my descent.
Its approximate, but its pretty close for planning purposes.
Its always just:
[Altitude Drop] / 500 = Minutes
Minutes * 2 = Miles