I change strings so infrequently, that I forget when the last time was, and what I used.
I keep a slip of paper in my guitar case(s), usually torn from a string packet, on which I write the date each time I change strings. I first did it out of curiosity, because I had no idea how often I "felt like" changing them (ie, noticed negative changes that might improve with new strings). My general usage pattern, with Elixir Nanos, tended toward string changes at 4 months. Having established that, it now helps me to be able to refer to the date of the last change if the strings start getting wonky, instead of trying to guess if I've gotten my money's worth or not.
I'd read all kinds of negative reviews on Elixirs in my pre-Taylor days, but I think that was maybe for the "soapy" Polys? Maybe not(!), but my 512 came with Elixir Nano 80/20s, and I really liked them. The PBs are by far my favorite, however. A few years ago a Taylor Road Show came through that featured Taylor Techs (Zach A., Pat Diburro-- booyah) giving guitars a once-over (and in some cases, doing neck resets, re-shimming and other exciting adjustments right on the spot) then installing a fresh set of 80/20 Nanos. I appreciated the gesture, but was eager to wear the strings down and out so I could return to PBs sooner than later.
So: Elixir PB Nano lights on my 512 and Baby (soon retired with the advent of the Mini), Nano PB mediums on my Mini (ahhhhhh), and Nano 80/20 lights on my non-Taylor-but-I'm-a-completist Guild dreadnaught, the boominess of which offsets and benefits from the brighter trebles the 80/20s provide.
Roughly every 4 months or so, sometimes 6.
That was the question, right?
~ S.