Steve I didn't do mine for 2 years (and I use the computer a lot!!!)

At first the hard drive overheated after about an hour of defragmenting it was that bad, now have a hard drive cooler (looks like one of them gay anodised halfords oil filter coolers ) and it took about 5 hours or something stupid like that. Computer was like new after though - it is well worth it! Makes it proper quick. Now run it every week to keep on top of it!