UPS battery back-up times can vary from 2 minutes, to many hours. It all depends on what you need.
The main question to ask before deciding on the UPS backup time is:
In the event of a long power outage “what do we want to do”?
Simply shut down the servers and wait until power is restored. In which case “how long does it take to shut down the critical equipment?”
Have a longer backup time in order to shut down a large database, or manually shut down certain applications. In which case “how long does it take?”
Keep key personnel, the telephone system or communications to remote sites operational. In this case a longer backup time will be required. “How long?”