How long will my session last?
In general you can say session.gc_maxlifetime specifies the maximum lifetime since the last change of your session data (not the last time session_start was called!). But PHP’s session handling is a little bit more complicated.
Because the session data is removed by a garbage collector that is only called by session_start with a probability of session.gc_probability devided by session.gc_divisor. The default values are 1 and 100, so the garbage collector is only started in only 1% of all session_start calls. That means even if the the session is already timed out in theory (the session data had been changed more than session.gc_maxlifetime seconds ago), the session data can be used longer than that.
Because of that fact I recommend you to implement your own session timeout mechanism. See my answer to How do I expire a PHP session after 30 minutes? for more details.
If session.cookie_lifetime is 0, the session cookie lives until the browser is quit.
EDIT: Others have mentioned the session.gc_maxlifetime setting. When session garbage collection occurs, the garbage collector will delete any session data that has not been accessed in longer than session.gc_maxlifetime seconds. To set the time-to-live for the session cookie, call session_set_cookie_params() or define the session.cookie_lifetime PHP setting. If this setting is greater than session.gc_maxlifetime, you should increase session.gc_maxlifetime to a value greater than or equal to the cookie lifetime to ensure that your sessions won't expire.