Best thing to do is initiate some kind of condition based maintenance. The problem is that you want to catch the potential failure before it is a failure. A few of the failure modes could be ( just brainstorming here): normal wear, dry running, hard object through vanes, abrasive sand in water, chemical attack, set vanes, hardened vanes, or increased wear due to higher discharge head.
Ideally, you'd pick an initial periodicity to change the first time. Observe the wear, and adjust the period for the next replacement. You could reinstall the impeller you just removed, but it can often be a chore to replace, so once your in there, you might as well replace.
For example, I changed mine after 12 months. It looked great. Changed the next one at 14 months. It too was fine. I kept adding two months each time. I am now at 22 month intervals. I am noticing that the fins are taking a set though. It will probably settle out at 24 months. I would probably decrease the periodicity, if I found sediment in my strainer, if I started and ran without my suction thruhull open, or if my running time hours increase significantly.
Did I save a lot of money or time? Nope. But I did decrease the potential for infant mortality, and I got to learn a little about my boat systems. I changed out screws to Allen head cap screws too. Makes it easy.
For those of you who change annually, good on ya! Periodic maintainance works too.
