Wow...so much unecessary work! First off, a complete full install of Windows 7 takes about 10-15 minutes when using a flash drive. Then, the rest of the driver installs, updates, and software installs will take about 2 hours or so...so if anyone wants to claim that it takes them much longer...it's time to stop overtweaking and get organized.
2 hours just for a browser install and customizing??? So far irrational that it doesn't even need to be commented on.
As others have said, Windows 7 needs very little done to it by default, unless you feel like tweaking things that shouldn't be tweaked.
I see it as breaking down into two options. First, you realize it doesn't take that long to do a clean install, and you learn to organize your necessary drivers and apps so then can be done quickly, and you learn to leave out the needless tweaks. The second option is, if you really need to hide behind the "it takes me too long to do a clean install" view, then you should have a good, prinstine image configured with all of your *ahem* tweaks, read the be reloaded in 30 minutes or so. Right?
How much sense does it make to complain about how long it takes a person to do a clean install, and then not be keeping system images? That's like complaining about getting wet in the rain when you refuse to use an umbrella.
I have two flash drives. One has my combined, all versions both platforms of Windows 7 to handle the install. I have a second flash drive that stores my latest drivers and software. In two hours I can have my full system back up and running, fully configured. The OS customizations take 5 minutes or less, because very little needs to be done to Windows 7 in order to have it run at peak efficiency. Anything more and you are likely degrading performance and risking stability. This 2 hour window covers my home tower, my work tower, and my work laptop, all with much different software configs and set ups. Why make the process longer than it needs to be?
To answer the OP's question, a clean install should be done when nothing else solves the issue, and shouldn't be done on a schedule. This isn't XP and too many people assume it is the same, where it needs to be reloaded or tweaked to run better. My work tower has been running since Windows 7's release on August 6th, 2009, and hasn't needed a reload at all. It's in use for minimum 8 hours a day.