Re: How often do you restring?
The answer is, "When I need to."
I adhere to a strict string cleaning regimen, so my biggest issue (besides tone) is grooves in the strings from fret wear.
I use Fender Super Bullets (9-42) on my G&L guitars with the DF Vibrato, and D'Addario XLs on my Gibson and Ibanez electrics, in a range of gauges from 9-42, 9-46 to 10-46.
For my acoustics, I use D'Addario Phosphor Bronze J-Series or their coated EXP sets; or Martin PB SP or their coated Lifespan SP sets. I mostly use Medium sets, but a couple use the D'Addario hybrid Bluegrass gauge and I keep Lights on my Ibanez Artwood. I use the coated sets on guitars that I'm not playing often, and the uncoated strings on guitars that I know I'll be using solid for a week or two.
I find that acoustic or electric, strings last me about 12-15 hours of "GIG" time, and about 20 hours of "SOFA" playing time. Last summer I was doing a one-night-a-week solo acoustic gig, three hours per night. I used my Martin HD-28E Retro (Mediums) and a Takamine 12-string dread. I used EXPs on the Tak, and didn't change that set for the whole summer, since I was only using it for 7-8 songs per night. With practice on the Martin during the week, I changed those after every two gigs.
The 80/20 Brass strings go dead on me much faster, even the coated strings. So tonally, they might get only 10-12 hours max. They are brighter than the PB strings, but they don't hold that tone.
One other tip I can pass along about string changing, is to let them sit for 24 hours, if at all possible before playing them. They will hold their tune better, and keep their tone longer. String it up and let it sit.
And the real trick, is to always change your strings before you NEED to. Kinda like...underwear.
Bill