I don't like the sound of brand new strings - I leave them on until they're just starting to sound dead and then I change them. There's always a cloth beside me when I play so dirt isn't a problem. It usually works out I change strings every 2 - 4 months depending how long I play each day.

I don't keep track of it, but probably about every three months. If I see corrosion forming, as does from sweat in the summer, I'll change more frequently.

I think that the sound difference exists, but is minimal. I don't change them for sound improvement, but I always fear breaking strings in the middle of a gig. If you do a fair amount of bending, look at the back of your old strings when you remove them and see how the fret has worn a flat/groove in the string...a likely place to break in addition to the bridge.

I have seen accounts on TV that many of the big name touring pros have their strings changed prior to each show. Other than the $4/set (in volume), the only disadvantage I see in new strings is that you must be sure to stretch them so they will stay in tune.

_________________
The quintessential sound of 60/70's R&R:
Fender Tube Amps
Gibson Guitars