Pistons can normally use 3-5 times their altitude to be lost. Jets should use a slightly larger multiple (jet folks chime in).

Ex. 1

FL24 down to FL18 = need to lose 6k.
6 x 5 = 30 nm as your TOD (top of descent point).

That rule is good for rough calculations, but sometimes you want to descend at 500 fpm rather than guess. Then use this:
2 times your GS in miles per minute in the descent x altitude to be lost in thousands.

Ex. 2

If speed in the descent is to be 120 kts over the ground, then 2 nm-per-minute and you want to descend 6000 ft:
2 x (2) miles per minute x 6 (altitude to be lost) = 24 nm