I never replace strings before they break, but I use thin strings (Ashaway ZM62) so they always break before there's any appreciable loss of performance.

How long they actually last me is quite variable. Gradual wear and tear doesn't seem to be an issue – rather, the strings go from perfect to broken in the space of 1-2 mis-hit smashes. If I'm playing well they'll last me a long time; if I'm having a bad day they won't make it through the first warm up after a re-string.

My advice to you, would be to consider how often you're prepared to re-string your racket(s), based on your budget and the amount of inconvenience it causes you – and then experiment with strings until you find one that will last you roughly that length of time. If your current strings are lasting you 4 months, they're probably very thick, and designed for durability rather than performance (eg. Yonex BG65/BG55). If you switched to a thinner string, it would likely give you *much* better performance, but still “acceptable” durability.