“how long is considered okay/mentchlich for the girl’s side to take before getting back to the shadchan with a yes or no?”
Technically, as long as you feel is necessary. Practically, in todays warped world of shidduchim where the females are “yadam al hatachtona” as quickly as possible. You wouldnt want “the best bachur in the yeshiva” to simply move to the next name on his list, because he can. He (more accurately his mother) can take weeks to “look into” a girl, but chas vshalom his highness (or more accurately, his mother) must wait more than 72 hours for a yes fro a girl!