The 3 times table works wonders here, which is of course based on a 3 degree slope, 318 ft per nautical mile.
3 times your height above the ground, for example, FL390 would required 117 miles, plus the speed you need lose, typically descending at 300 knots, you need 1 mile per 10 knots, so coming back to clean speed, will need about 10 miles. Also take into consideration the wind, usually a mile per 10kts of wind adjusting for a head or tail wind.
All this stuff will come with experience, and you learn quickly what your aircraft is capable of.