UPS systems are generally configured for 90 minutes of operation, depending on the criticality of the system they're connected to. The best ones are programable and will actually send graceful shutdown signals (when configured to do so) to your server cluster to prevent data loss that occurs during system blackouts. You can emulate this behavior on your laptop with a script that checks battery% every 10-15 minutes, sending a shutdown signal if it falls below a treshhold you set.