Time until threads marked as read

How does the time-out thing work? Some days I can spend a long time searching the web for an answer and threads don’t get marked as already read, but other times (today, for instance) just a couple of minutes away and all the threads were maked as read. The ‘last visited’ thing gets reset too.

This is what I could find:

This option controls how threads and forums are marked as read:

Inactivity/Cookie Based - once a user has been inactive for a certain amount of time (the value of the session timeout option = 900 seconds) all threads and forums are considered read. Individual threads are marked as read within a session via cookies

Ah, 15 minutes to look elsewhere. I’ll have to refresh to keep my session open…