How long does a domain name take to become available to purchase after it has expired?

It depends on the domain registrar (the company used to purchase the domain). My webdev company does *some* monitoring of domains for our clients, and some registrars offer the domain for sale the minute it has expired. Other companies wait until the end of the day, and others still give the customer a day or two.

It should also be said that there are several services (both affiliated with the registrars and completely separate entities) that exist to snatch these expired domains up. If a domains is about to expire, it will alert interested parties to it, and sometimes even broker a possible buy-out offer from the interested party to the domain owner.

Depends upon the registration company. Give you an example: I have had some half a dozen domains with GoDaddy.com. Sometimes, GoDaddy will extend the registration for like 3 months thinking that you will come back. Some other times, if your domain is pretty good, they're going to hold on to it for their own customers (so you can only get it from a certain company).