No telling. In my experience it's been anywhere from 30-180 days, mostly depending on the title search.
Given that, you can always ask to take possession prior to close. Means the place won't be setting empty while everybody waits on paper-shufflers and rubber-stampers to do the job they get paid to do.
Offer the owner a tenner as token rent until close. I do it all the time when I want to get started remodeling a new property and don't want to wait until closing. You can insure the place as a renter, and the previous owner gets to write off a 'loss' as the landlord.