How do you know when its time to replace your engine?

[YZFr1]

Hey guys,

Just curious to know, when do you know when its time to replace an engine? Like is it at a certain mileage?

also, curious, roughly, how much would it cost to replace a d16y4 with a d16y4 on a stock ek1 hatch.

also, would you know roughly what kind of mileage the replacement engine would have (range)?

cheers in advance