I don't think there is a rule for this, but if you have ever seen a small bird blown off a branch due to an unexpected wind, you'll notice that they recover pretty quickly.
I would suggest something like the 5 feet fall of the Ring of Feather Fall where the flying creature recovers and then flies up.
Less than 5 feet, maybe a +2 synergy bonus to the Reflex save for falling in.
This could also be dependent on circumstances. For example, a narrow pit where a flying creature with wings could not spread them out might result in a lower initial Reflex save, but if you miss the save, you cannot fly out or something.