There is no entry window. Once it appears, you are free to send in your entry. Every time I've put on an event, there is usually a small number of very keen riders who seem to have some sort of alarm system set up that notifies them when events go live.
My first entry for today's ride was literally 2 days after it was added to the calendar. I've already had my first entry for a February ride (suspect PayPal made it too easy for them to resist, mwahaha). For an organiser, I find that the first entry is a combination of relief (ah, at least one person will be doing it) and dread (damn, I actually need to do this now).
Personally I much prefer receiving early entries than those close to the wire. Those later entries can be a pain in the proverbial (although my own fault, because I tend not to impose a closing date and usually allow entries on the line).