I have a 30gb and after installing windows 7 64bit I had 15gb left, barely did any updates though, I recently decided to change, so now I’m trying out windows 10 enterprise, again 15gb after installation, after some updates and couple programs I have like 8.??gb left, it’s the only drive in the system as it’s original intended use was general, but main focus around movie watching from my mini projector, so I would say it’s enough for the OS, more so if you setup page file or whatever differently as I don’t have a 2nd drive to test that.