Unless you are already on top of the airport at 8000ft after forgetting your descent, you usually have a pretty good idea of your desired rate of descent. On unpressurized aircraft it is usually something like 500fpm for passenger comfort, on pressurized aircraft 1000fpm or more;
So if you need to loose 12000 ft (cruising altitude - final approach altitude) with no ATC restrictions and you are in a unpressurized aircraft, then you would want 500fpm, thus 120000/500= 24 minutes out. This works if you don't increase speed and make the descent by reducing power.
Couldn't be simpler and takes into account head or tailwinds since you're working with ETA.