It really varies based on your location. In my area, it is normal to charge your full time rates monthly, in advance. My part time rate is based on a daily calculation (# of days x $40, paid at the beginning of the month). My drop-in rate is the same amount daily, but does not have to be paid in advance. Drop-in is dependent upon space available, and (to be honest) whether or not I feel like having one more that day.