This is only if you desire to get the honorarium (for yourself or for the committee), and the limit is 240 hours before the event. This is designed on purpose to 1) give the calendar admins time to approve the class, and 2) allow enough advance notice of the event on the calendar to give as many people as possible a shot.
As for no-shows: I attach a $5/person "fee", which puts a little skin in the game, with a no-refund window of 24 hours before the class. If someone doesn't show, I open that spot up to the standby members, who pay no fee (since someone already did that for them).
(Edit: I guess I didn't address your actual point -- yes, unless you anticipate classes filling up, there is somewhat of a time quanta between honorarium classes).