Rest is always a good thing. However, if 18 days is optimal or not will depend on the type of training you were doing before the SD, the amount of time since your last SD, training age, etc. etc. The harder your routines. the longer your prior training period, the older you are, how long you have been seriously treining, etc, will all tend to lengthen the optimal SD period.
Rule of thumb:
When in doubt, lengthen SD's and shorten workouts.