In app/config/session.php you have:
option that allow you to set session expire time in minutes (not in seconds)
'lifetime' => 60,
means that session will expire after an hour.
There is also one more setting here:
'expire_on_close' => true,
that decides if session will be expired when browser will be closed.
Other settings you could get interested is also php.ini values of:
session.cookie_lifetime = 0
session.gc_maxlifetime = 1440
Those are default values.