As with most thing, it usually depends on your regularity of playing and how much you do play when the time arises. With the bow, I’d usually recommend getting it re-haired at least annually if you’re playing regularly. Of course, sooner if you notice issues with playing, or if you play heavily so that hairs are regularly breaking. As for string, that one completely depends on the type (steel/gut/synthetic) and regularity of playing. I’ve seen strings have a very long life and short depending on the details. What do you have and are you noticing anything unusual happening, or serious trouble with a string tuning up properly?