MS reckons less than 10% fragmented isn't worth bothering with so that's probably a reasonable benchmark to use. Either use Defraggler to analyse the drive or at an elevated privilege command prompt:
defrag.exe c: -a
I'd almost forgotten about defragmenting drives - I just let Windows get on with it on Mme Pingoiun's laptops. Might get checked a few times a year if the laptop is on in the wee small hours. But then the drives are only half full. Entirely forgotten about on Linux, even with a very full drive. Used to play with it all the time with Win98 on FAT32
Curiously defrag.exe reported a Vista drive as 5% fragmented while Defraggler reports 18%. Something to do with what Defraggler calls "low occupancy" perhaps?