Deleting a lead is very similar to Archiving a Lead in Open Leads. Exercising any of the options to delete a lead, whether it be from the Lead List (as a mass delete or single delete) or from within the Lead Detail screen itself, places the lead in your Lead Archive. The deleted lead will remain in the archive for 30 days, allowing ample time for an admin to "undelete" a Lead that was accidentally or erroneously deleted.
At the 30 day mark, a deleted Lead will be permanently removed from the archive and cannot be retrieved.