A few months sounds suspiciously short, but that's a really open question. Depends what you play, depends how often you play. Depends whether you're using ultra-traditional catgut strings, or some synthetic ones which will be longer-lasting. I understand that some people have more acidic hands than others, which will work through strings more quickly. Tension and tuning matters as well; I'd normally replace my violin strings every year, acoustic guitar every other year, and my electric guitar like every five or something, and that's probably more often than needed for the electric.