You should replace your UPS batteries once they have reached a certain age or a clear reduction in available run-time is evident.

Depending on the model of your UPS it may or may not conduct regular checks on the battery capacity.  If it doesn't, how will you ever know whether the batteries have the capacity to support the connected equipment for the required time.  If it does, how long does it test for and does it give a good indication of capacity.

Some UPS units test their batteries for 10 seconds every 2 weeks, and about all that proves is that the batteries are sufficient to operate for 10 seconds, not particularly helpful if your PC takes 30 seconds to shut down.

Replacing a UPS battery based on age

If you've already spent money on a UPS, its a fair bet that you consider losing power to your PC is too great a risk.  Why would you then risk losing power to your PC by using a battery right up until the end of its life.  For this reason we say a system under ideal conditions should have the batteries replaced every 4 years, and a system in fairly ordinary conditions with high temperatures and lots of discharges every 3 years.

Replacing a UPS battery based on performance

From new the UPS battery is losing capacity, it starts slowly at first then gradually picks up the pace until it gets down to 80% then after that it drops off very rapidly.  The best way to determine your battery capacity is to perform a known-load test.  Rather than relying on the internal battery test, choose a time that suits you and turn off the power to the UPS.

If you have software or the UPS has a display you can keep an eye on the battery capacity / run-time available, if not most UPS will beep differently once they have reached a pre-defined 'almost flat' setting.  Take note of the time taken to reach a certain point then turn the power back on and let the UPS recharge.  Conduct this test every 6 months or so, and as soon as you notice the run-time shorten significantly it is time to replace the batteries.