When to replace turntable belt?

I usually check platter speed with a simple phone app that also gives me a Wow reading and I save them. If I see that Wow reading start to get worse then it's probably time to replace the belt. As noted they are inexpensive so every 12-18 months is a good time for me. But this also depends on how many hours a week you spin records.

Great info above. Currently comparing my OL flat belt from it’s life being shut on/off on each album side to..lifespan from listening sessions which  the platter is spinning continuously

The belt on my previous Clearaudio Innovation is about $100, but I just replaced it every year. I upgraded to a Master Innovation a year ago. Same belt, but it came with two so I am off the hook for the next year.

I suppose I could get by waiting longer, but I play my turntable on average 2-4 hrs every day (more in winter, less in summer), so I figure changing it every 1000 hrs or so of use isn't a bad thing to do.