I believe its between 15GB-20GB stock. Take into consideration page file, temp files, hibernate files, etc.
I installed windows home prem. on a 64GB SSD, with drivers, and main programs (except games) and I'm currently at 28GB use.
I used this page --> http://www.sevenforums.com/tutorials/11728-optimize-windows-7-a.html?ltr=O
and pick and chose options, one was step 13: Turn off system restore. This will free up some space.
And also, a recommendation from Mod Nikorr I just received, they mentioned that you can get the OS down to a whopping 800MB using RT se7en lite. Props to Nikorr for that recommendation.