Manual:$wgObjectCacheSessionExpiry

Details
Expiry time in seconds to use for session storage.

When a user has logged in without checking the "keep me logged in" option, they will be logged out if they are inactive (do not interact with the server) for this much time.

For "keep me logged in" logins, the session will still be discarded, but the user will remain logged in as long as their authentication cookie (controlled by ) is intact.