My UPS manual says 3 years. They have a different chemistry than phone batteries. On the other side, they should be quite similar to car batteries which could live much longer that 3 years, but they spend most of their time waiting in a parking slot and then a few starts per day or less. An UPS doesn't work like that.
A good UPS will perform regular load tests to gauge battery performance/degradation. They switch over to battery power around once a week and measure voltage before, during, and after. So even if the power goes out, they are still being exercised _somewhat_.
