When does steam stop counting gameplay time hours?

I just finished recording a full 15hours of gameplay of bully but steam didn't log the time played correctly (in-game stats said I have 15 hours).

And the weird fact is it is inconsistent, my online profile says 13.1 hours, steam client says 12 hours

Back to the main question, when does steam stop counting hours played?:

• No Internet Connection/No Connection to servers(steam is offline but Internet is plugged)?
• Chat status as "Offline"?
• Steam offline mode?