i just checked mine. I have 10.77 GB left after downloading all of my Apps (1.08 GB) and other things like pictures, videos etc. (not that much). So my guess would be that after the OS there is about 12GB left. I obviously have the 16GB model. As mentioned below, stock Android is VERY light weight and it's one of the reasons I love this tablet. OS is sleek and fast.
I had the first nexus but go frustrated with Android, as it was my first real experience. I am curious to go back. My main frustration is the lack of high quality tablet apps and the odd fragmentation caused by the processor in an Android device. But I am curious....
I looked for numbers but didn't see them mentioned. Considering how light weight stock Android is I'd imagine on the 16GB model it'd be at a minimum 10GB free, but more than likely over 12-13GB. If you can get into a store that has one on display, I know my local BB does, I'd just pop into settings and take a peak. It wont be fully accurate because people probably downloaded stuff but you could get a good idea.
Engadget reported 26.1 GB available on the 32 GB model.