Well a fully updated Windows 7 backup system image with a few extra apps takes up about 17GB and takes about 10 minutes to re-image a hard drive at which point I would usually update everything and do a new backup image which takes 15 to 20 minutes. Considerably less than the 3+ hours it takes the normal way). With drives the size they are nowadays, it's not exactly occupying all the space. There may be easier ways to do it with Acronis or something that take up less space but for me it works and it works well.
I imagine that Windows 8 will take up around the same give or take a GB or two.