suggested mileage for piston replacement?

 

Normally if it's not broke don't fix it.

It depends on oil blow by do you see blue or white smoke on start up? Are your plugs caked with oil?

You can do a compression test and see if you have low compression. If you do then do a leak down and see if your valves/rings/head gasket is leaking.

 

when they are worn beyond factory replacement specifications. Are they out of the engine?