Session, timeout question [Help]

Hi all,

I have set the session timeout from 120 minutes to 20 mins. However if I stay logged in and then either go to another tab or don't use the browser, this should log me out, right?

However, I don't always see this happening, and I was trying to complete quite a long form on page, whilst doing other tasks away from the browser, so my question is; Is the "Idle" time from one request to another request? i.e clicking another link or a button on a form?