5 - 30 minutos
I had an 80 gig drive, so I was getting a start up disk full constantly.
So, so easy. The steps were all easy to follow. The 80 gigs took about 2-3 hours to back up on the drive. The physical install took about 20/30 minutes and I was up and running right away. Some files needed to be remapped to what programs open them properly (mostly doc and xls files), but otherwise it was seamless. And now I have more hard drive space then I know what to do with.
Do it. If you are fearful, don't be. Especially once you are no longer covered under Apple Care. There's nothing to worry about voiding. You'll be so happy you did.