Depending on your skill level installing Windows can take between 1 and 3 hours. Could take the better part of a day to get all your software reinstalled as well as your files transferred from your backup.
You might want to also make sure that all needed drivers are available on hand (on a CD) before starting. This would save some extra time and headaches afterward. Best thing is using a free 3rd party software to backup all the current drivers and save them to a CD.