When they stop going ting and go thunk instead. I've tried Elixair strings but the third never lasts more than a week, At $25 a they're not worth it to me. Martin Darcos 80/20 's work well on both my guitars. D'addario EXP42 for dobro and Martin Mediiums for banjo last a quite a while with out a lot of breakage. There are a lot of good dulcimer sets too. Martin seems to be the most available. I usually get several sets at a time which gets me a good price break. Even then you might get a batch where one of the same string is bad. and it continues thru the whole case. I've even had strings missing.
But dealers are always willing to help you out in replaceing the problem. Now if we could only get harmonicas to last!! Cheers-Dave