Ruben is correct, it will stay in for 24 hours. If the email isn't verified in that timeframe, the next time someone goes to register a user, it will be deleted.
So, that's the current trigger. If you don't have anyone register for 3 days, then it will stay in there for 3 days even if it's unverified.
In version 8, this will be moved into the scheduling system, so it will be more consistent.
