It's almost an oxymoron, as most computer based UPS's do have a limited life span. What determines how long a UPS can support, is simply the load you apply. Today's new computers (the gamers unit), is a burden on the battery, as are the older tube monitors. I've been using UPS's for many years now, and depend on then 100%, not for just power failures, but brown outs (when the voltage dips below 120), and I live in an area which supplies clean power. The other added feature of any UPS, is the ability to prevent surges from damaging the delicate units. The Multi-function units also known as printers, can cause a surge when starting (so never plug then into a UPS). Interesting!!!
One of my units, an APC model 1500, can supply power to 2 running computers and led monitors for about 45 min. (with a load of 250 watts on the battery). Just operating one computer and monitor for over 80 min., Mindblower!