Those are the million dollar questions!
Every Registrar has their own rules on how long they hold a domain name past it's expiration date.
There are domains with expiration dates as far as 1998 that have not been released by Network Solutions for re-registration. Many think NSI is squatting on these domains (generally they are attractive and valuable domains) so they can auction these domains eventually.
Since NetSol is obviously the largest Registrar, I can offer some insights into their practices based on our current experience with them.
1) there is no rule/regulation/law that dictates they have to release expired domain name within a specific time frame - this is a leaving from the times when NetSol had sole monopoly on registrations and they had self imposed rule to expire and release domain in about 65 days.
2) 65 to 85 days rule still sticks to most domains. NetSol gives domain owner extra 30 days after domain expiration... just in case the owner is in Europe vacationing and could not send off that check on time. At 30 days past expiry date NetSol sends a nice letter with Past due notice and they give you another 30 or so days to pay up. On 60th day or so they would de-activate the domain so you no loger see your website serving or email arriving and NetSol sends you this nice Deactivation Notice. Then on about 65th or 85th day past expiry date NetSol deletes the domain from their registry and makes it avaialble for re-registration to other parties.
This process is NOT described or prescribed anywhere - it is just what we observe as most common process out of NetSol's offices.
Mysteriously, some domains do not get re-released. Some may be in arbitration - that is if someone sues someone else for that domain name - NetSol is notified and they place a HOLD on that domain name.
We are keeping a close watch on these proceedings. NetSol will not disclose when any particulard domain will be released for re-registration even if the domain shows 1000 days past it's expiry date. The only time we have had a hearsay report from attorneys who handle Class Action against NetSol for hoarding expired domain names - NetSol's Attorneys talked about the 65 day rule during the court proceedings.