Belt Tension and life expectancy.

I have a Lulzbot Mini that has been in use for about fifteen month and consumed abut five full reels of PLA and HIPS. The prints are not as accurate as they had been and I suspect there is some wear and tear on the mini. I tightened all the screws, cleaned all the smooth rods and put a light film of lithium grease on the Z screws. I also tightened the belts, but there was an occasional clicking sound during prints done after all of this maintenance. I loosened the belts and the sound went away, but the print quality is not as it was before. The maintenance guide refers to the belts being tight when pressed in the middle but does not have any details. How many millimeters of deformation when the belt is pressed in the middle?
How often should the belts be replaced? Is there a time limit, material limit, or stretch test to determine when the belts should be replaced?

If the belt looks cracked or frayed, it should be replaced. No mini on the planet should be anywhere near belt replacement age yet though. My old AO-100 belt is still fine. For belt tension, it should make a sound like a plucked bass string when it is tight enough. You shouldn’t be hearing a clicking, but that can indicate other things (gunk in the bearing, a bearing that is going out, an idler bearing or motor bearing that is failing, or in some cases the dreaded mini motor shaft snap issue). Belt tension is particularily important for circles. Print a circle calibration object. if the circles look squished in one direction, that’s the axis you need to tighten the belts on. If the circle looks fine, your belts are fine and tightening them more shouldn’t help more.

When you tighten the belts make sure the end of the belt is not hitting the teeth on the belt as it moves back and forth in its full travel distance.