Marking cams, give the indication of when strings are stretched. Usually a few twists on the string, orcables will correct this. Not waxinga string and cables will create wear, afuzz(hairy string).
Most often just looking at the string one can tell if it needs to be replaced. Busted strands in a string is certainly a reason also to replace a string. Some guys have better luck with their string and cables than others. 'If' the cost of replacing the set isn't that of a problem and the bow is used all thru the year and not only during the hunting season, I would suggest the string and cables should be replaced once yearly.
Otherwise if the string 'appears' to look in good shape and the bow isn't used as often--and supported with constantly waxing the string you can geta littlelonger life out of them.