This answer mentions that Zvezda has S5.79 engines for reboosting the ISS. Wikipedia mentions that this engine has a burn time (ie. lifetime) of 2700 s. Dividing that lifetime by Zvezda's age (20 years) gives around 2 minutes of burn time per year. I think that station reboosts require longer burns, which would use up the engines faster.
Are the engines designed to be replaced in orbit? Have they ever been replaced?