As long as the registrar decides to hold it. Some registrars hold on to a domain while they try to convince the former owner to renew the registration.
I've seen GoDaddy grab good domain names after they expire and add them to their domain auctions, so even after the domain is released, it may not be released for direct registration.
The longest I've had to wait to pick up an expired domain is about 3 months.
Be sure to check out its prior use thoroughly. Penalties applied when spammers used a domain name for a crappy site are sticking with the domain. There are very likely hundreds of thousands of tainted domain names like that.
Penalties are Sticking With Domain Names
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin