It varies a great deal. Each team has a distinctive culture, and there's a lot of individual variation too.

I typically arrive at Google about 9:20 (until recently the breakfast cafeteria at the NYC office closed at 9:30, although that's no longer the case I've kept the habit), but don't really start working until ~10. If I have plans in the evening I'll leave around 6, if not, I'll stay for dinner and keep working until 8-8:30. I've never come in on the weekend.

One coworker on my team is always there before I arrive, and is always still there when I leave, but everyone else on my team floats in gradually until 11:30 (when we have our daily standup), and leaves sometime between 7 and 10.

While some days I may spend 10 hours in the office building, consider that this includes 3 meals and usually at least one other major distraction (a tech talk, a break in the game room, TGIF, or something).

Edit: I've been in the NYC office around 4 months.