its actually about 6gb(+/- 2gb)
as far as how far will the 8gb go, I think it would be better than what people expect. Honestly most of the space on my Gnex is taken up basically because I just don't delete uneeded zips/apks, and backups. Keeping a bunch of backups ON your phone is a bad practice as is, you should always transfer those to something else with the storage being fixed and non-removable. In the event something does happen, your kinda screwed with the backup only being on your phone. Its fine if you keep 1 backup, but anymore than that usually isn't needed. Same thing for rom zips, I have no reason for keeping versions 8.0 on up of jellybelly along with all the different gapps, but i do simply because i just never bother to delete them. The rom files with gapps alone is probably 2gb+. I I know not everyone uses streaming but I do, so music isn't an issue for me when it comes to space either. Would I like more space? Yes, but I can live completely normal without it, I just have to not get messy with what I store on it.