Well, it varies a lot, depending on your playtime and style (pick, fingers etc) With the acoustic, where I use a heavy pick I wear off the coating under my pick hand after about 3 months, and they start to get dull. With the electric, where I play with fingers, and rotate between 4 different electrics, I usually change them about every 6 months or a bit more.
String life is not just a matter of oxidation, and coatings to prevent that. Taller frets tend to render poorer intonation over a shorter time frame, because the strings get bent over the tall frets. The string itself may be spotlessly clean, but if you pluck it you can see it's not a "clean" wiggle, but more of a wobble.
I actually like the slight slippery feel of the coatings, I didn't like the first generation (polyweb) but both of the newer ones feel nice. The worst "long life strings" are stainless strings IMO, they are so sticky and chew up both your fingers and frets.
Nickel flats last forever. Tomastik Infelds as mentioned above have a midrangey thing going on that some may not like. I have liked them on some guitars before. Now running D'addario Chromes on my archtops, and will try them on my flat electrics next time. Also like Pyramids but the price; soundwise kinda in between the other two.
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