21,600 Threats? Wow most AVG should normally find is around 15 on a bad week. Luckily AVG should be able to fix it all, even if it has to finish upon restart.
As for how long? Depends on amount of data on HDD, and how fast you set it. My 320+160GB HDDs take about 2-3 hours due to having very little stuff saved (around 150GB used) and I use "Slow Scan". The 2-3 hour is a guess, in that I mean i start it between 1am and 3am and it is finished before 5:30am.
Another thing to do once the PC is clean is to set AVG to run sometime when you are not using the PC, and if wanted set it to turn the PC off after scan.
I have only had one threat while using Mozilla Firefox, Spybot S&D, Ad-Aware, AVG 8.0 and Windows Firewall (I know, the devil) in about a year.
So if done properly a PC can stay Virus clean very well.