Hmm...I don't have much. I have heard it takes anywhere from hours to days for flowers to replenish. It probably depends in large part on the flower too. A short lived flower won't be as affected as one that lives for days, is my guess.
I think one hard rain will wash it out, but there will be forage within a day in sunny weather. Keep an eye on the hive, if they look like they need the extra super, go for it. If in doubt, I always keep one or two extra on the hive. I'd rather have an extra super on than have them run out or space.