How long is long?
Some people keep them till they break, but i think once every month or two, depending on how much you play. Apparantly professionals change every week

Strings lose intonation, probably due to fatigue of the metal
Other thing is they can get gunked up with sweat and old bits of skin
People say you can clean you strings is boiling water but ive never tried it
Whats the point in boiling guitar strings when they are so cheap? i know some bassists boil strings to give them a bit more life but thats because they are more expensive.

I've never really had any major problems with innotation so I don't know.
Guitars Fender Highway 1 Telecaster, Vintage VS-6, Yamaha C-40 Classical
Amps Laney VC15, Crate GT212 *FOR SALE*
Pedals Fulltone OCD, Vox V847, Marshall The Echohead, EHX Small Clone, Korg DT-10

Member of the Laney Cult
Corrosion and finger grime change the vibrational pattern and capability of the strings. If they can't vibrate at the same rate and the same pattern the intonation goes out the window.

I change strings whenever necessary at home just practicing, for economic reasons. 10 guitars would keep me broke if I changed them as often as I should. When gigging the ones with whammy bars get new strings every gig, guitars without whammy bars get new ones every other gig, the lap steel about every 4 or 5 gigs. I hate breaking strings onstage and I play hard, if I try to use the same strings two nights in a row on my Strat or Peavey Patriot, both with whammy bars, I will break strings so they get new ones every night I play, period. The others don't get so much abuse and don't get worn at the saddles as bad and usually last 2 gigs...
Hmmm...I wonder what this button does...