When they break, or when I feel like it has been too long.

Actually for the optimum tone if I am playing with a band regularly I change the strings on my electric every 2 weeks, and the strings on my acoustic once every 2 months

Depending on how much I play, usually 2-4 weeks on my main guitar, though I have found that SIT strings last quite a bit longer than others.

when one breaks lol or they get dull..

I should be doing it once per 2 weeks, but I end up changing them every 2 months...