How long to wait: Depends on the area. If you are having a shopping site and you know the competition is hard it may well take some years, since the amound of submissions is so large. In less active areas it may take some days. There is no better advice to give, sorry.
What to do: You could ask in the "Submission Status" forum if it is still waiting. Dont ask to often to save us some work.
Know its rejected: Only by asking.