Obviously upgrade times are heavily dependent on database size and data tier performance. Here are some examples of upgrades I have run.
Upgrade from TFS 2008 600GB databases on high-end physical AT and physical DT: 8 hours
Upgrade from TFS 2008 250GB databases on moderate DT with external USB hard drive and AT VM: 25 hours.
Upgrade from TFS 2008 50GB databases on moderate DT, and separate AT: 1.8 hours
Upgrade from TFS 2008 50GB databases on moderate DT also hosting the AT VM: 3.5 hours
Upgrade from TFS 2005 would be a bit longer than TFS 2008 since there are more steps to go through.
Upgrading from 2010 Beta2 are 5 times faster than upgrades from 2008 since there is much less work to do.
Upgrading the 600GB dbs listed above from Beta1 to Beta2 took about 1.5 hours, for example. I expect Beta2->RTMwould be similar.