Well it also depends on how many files you have on your computer. I for one have around 3 MILLION files so it takes 10+ hours to scan with AVG. In less than 2 hours I can have the thing reformatted and working 100 times better than it did before I got infected.
It really is a good idea to format every 6 months. I'm pretty lazy when it comes to this though so I do it about once a year.

Formatting shouldn't be looked at as a bad thing or something to do as a "last resort" but rather a healthy thing that your computer needs now and again to be at it's peak performance.
It just makes no sense to spend hour upon hour "fixing" the problem when you can spend less time by formatting and restoring your data. Even if you do "fix" the virus, your computer is still slow and full of junk. Chances are you only get 1 or 2 viruses a year anyway unless you're a porn junkie or something so why not use that opportunity to speed up your computer and get rid of the problems?
EDIT:
I should add that formatting your computer every 6-12 months can also help prolong the life of your processor, memory, power supply, and hard drive. If your computer is all junked up with crap it has to work a lot harder to do the things it needs to do therefore putting more strain on your components. Depending on environmental factors one of these components is more likely to fail than the other if you keep your computer junked up, but most of the time your hard drive is what benefits most from formatting.