For Win7 64-bit plus a handful of 64-bit applications this may be pushing it. I'm not running any Photoshop or games and I'm up to 50 GB. Win7 32-bit is, from what I have read, smaller.
So I say about 40, and others say 20, and I'm sure that cloaknsmoke is right, too. Wonder why we have different experiences. (disclaimer - that is a rhetorical question.)