There are a few considerations.
NPC sororities cannot try to recruit people before they would be eligible to participate in formal recruitment. For instance, if a school has deferred FR to the spring, sororities cannot rush freshmen in the fall. (This would not apply to sophomores and up. I don't believe this applies to new transfers either.)
Recruitment is not allowed over school breaks. For my school, this meant no recruitment during summer, between semesters, or during other vacations (spring break, etc).
Sometimes, when a new sorority is colonizing, the local Panhellenic will place a moratorium on COB recruitment to allow the new colony to recruit without competition.
A sorority that is both at/over total and has met quota cannot have an informal/COB recruitment. They must wait for the next FR unless they drop below total due to graduations/transfers/deaffiliations/etc.
There are more recruitment-savvy people than I on GC, but I hope this helps a little.