That all depends on what you mean by "signed in".

If your at a business or school that uses an exchange server, then you may very well stay logged in, and the computer could be locked.

If your referring  to being signed in to a specific website, then it all depends on how they store their sessions, some sites have their sessions expire after a certain period of inactivity, and some stay live for days.

It is always best practice when using a computer that has multiple users on it, to log out completely, and even to erase your user name if it shows up after you log out.