Let's say you have a aspx page that does not rely on session, but does rely on viewstate for persistance between postbacks.
If a user is accessing this page, and leaves for a long lunch, will viewstate still be valid when he returns?
Answer
Viewstate itself does not expire. Since it's posted back in a form, it can be reconstituted any time.
According to MSDN: "...it is possible for view state to expire if a page is not posted back within the session expiration time". So, in a round about sort of way, it can expire if your session does, but viewstate does not directly expire. Since you're not using session state anyway, you don't have to worry about implicit expiration.
Note that I wouldn't have said it expired. That was MS who I quoted in their own article entitled Controlling ViewState