I test my UPSes monthly. In general, I replace the batteries somewhere between 18 and 36 months. The smallest UPS eats batteries. There's also a huge difference in quality between the "best" brand I have and the "worst". They're certainly not low-maintenance items, whatever the manuals claim. For another location, I'm considering paying for the service warranty (which more than doubles the cost of the units) so someone else can visit once a month and do the testing.