And in a well designed one that's not either intentionally subject to massive amounts of power problems (notably of the 2-5 variety--sags, spikes, undervolts, or overvolts), the actual unit should last forever. If it's treated right (see below).

I have a location in the Dominican Republic (which is known worldwide for their crappy power grid) with a few of 10+ year old best power/powerware units that take daily hits on all 9 of the common power problems that have never quit and still test properly on the load side. The only thing that's ever been changed are the batts. These range in size and complexity from 1200-1500 VA units (line interactive units) in 90 degree F wiring closets to 2.5kVA double-conversion units in their small data center.

The most common cause of early UPS death is plugging a surge-supressor into the load side of the UPS. Only non-surge arresting power strips should ever be used.

I use Powerware 5125 and 9125 units in the 1500 VA range, and have never had the UPS unit itself go bad. YMMV. For home use, I like the Powerware 5115 and 9115 (the latter is overkill for most applications). Their 5110 is decent too, at it's price point (but does not have some of the advanced battery management capabilities of the 5115).