When to replace turntable belt?

It can also depend on the belt material and where you live. E.g., if the belt is made of a rubber compound and you live in an area with low humidity, the belt will likely need to be replaced sooner, since the material can dry out. Information as provided by the distributor of my Acoustic Signature turntable, where the belt is supposed to last 5 years on average.

The good news is they are not expensive and can last a fairly long time...so if you're thinking it might need to be replaced, then replace it...

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.