Well, they need to be on the island long enough to come up with an escape plan unless they've been doing that for a while. Maybe a few weeks or is that too long of an estimate?
EDIT: By the way, do we have our last child of Tartarus? If not -- and if it is okay with you, unworthy -- I came up with a concept for a young son of Tartarus and wouldn't mind creating him so we can start soon.