Without specific information, plan 3:1. FL240 to FL180 is 6000 ft x 3= 18nm.
to cover any other variables they might give:
At a ground speed of 420KTS that's 7miles per minute. It takes about 2.5 min to cover 18NM. To lose 6000ft in 2.5min you need to descent at about 2500fpm.