Yes, they are already in the config file, however, there is no distinction made between admins or clients in the session TTL.
Open your config file /config/blesta.php and update the session/cookie TTL values:
// Length of time (in seconds) that a session will be valid for
Configure::set('Blesta.session_ttl', 1800); // 30 minutes
// Length of time (in seconds) that a cookie will be valid for
Configure::set('Blesta.cookie_ttl', 604800); // 7 days