General rule of thumb is 2 years. If you notice any fraying on the main strings, get them replaced.
If you notice any of the single strings cut or in 2, don't shoot your bow anymore before you get new ones.
Generally serving around the cams and center will start to come apart or shift. You will start to notice your tune starting to get out of whack.
If you learn or are able to re-serve, that will save you another year or so maybe. Strings now a days are really good, but servings won't can can't last that long.