How do you know when the build plate needs to be replaced? Approximately how long do the last?
If you take care of it, the build plate will last you many years.
You pretty much shouldn’t have to replace it, but if you start having issues with prints sticking to the platform then maybe it needs to be replaced or you might be able to just sand it smooth.
I print a lot. Pretty much the only time I’ve thrown out a build platform is when someone has left it covered in resin out in the light and the resin cures on all over it. Even then, if I were more patient, I’m sure I could chip it off.
As the previous posts said, it should last indefinitely. If you have issues with prints sticking to it, simply sand the surface. Not too smooth, you want a little bit of grittiness. I used 220 grit sand paper, and it worked great.
I dropped a build plate and dented the corner. I repaired it by flattening the raised area with a mill file. It is now flat all across, as new, and works just fine. They seem to be very durable.