I pretty much just replace them when they break, i play about 3-4 hours a day and I only actually change all of my strings at once, about once every couple months.
When they start to sound off, if they have to much rust on them, if they brake obviously erm just when ever you feel you should really
Technically they say you should replace them every 4 to 6 weeks I believe, but that starts adding up to a lot of money, so I say screw that. When you notice a lot of dirt and crud piling up on them or the tone becoming dull...then you should replace them. Of course when they break you should replace them as well lol. Usually if you see rust, dirt, and crud building up you can replace them before they are ready to break though.
Quote by Shredlet
Cheers Sinister that was really helpful a Dime-point to you!

"Any problem you can't solve with a good guitar, is either, unsolvable or isn't a problem."
Depends on the strings too. Elixir's have to be changed about 1/3 less often then most others because of the coating. But, they are 3 times the money as regular slinky's or D'Addario phosbronze. I prefer to spend the money because changing strings is not something I like doing.
Light is faster than sound. That is why some people appear bright until you hear them speak.

Quote by Scott3229
if i wanted to get brain washed and then throw all my money away, id go to church

"I see, you've been blinded by what you believe" Sully Erna