I typically use the d'addario strings and go between PB and 80 /20s, and have started to just change them when they start to fail, not due to sound. It's typically the G string which goes first and I'd say 3-4 months in is when I'll notice a notch in it... usually at the 3rd fret. I feel the 80/20s fail before the PBs.
I do love the sound of new strings when I put them on, but rarely do I pick up a guitar play it, and say these strings need to be changed... I just try to make whatever i'm playing sound as good as possible with the magic in my hands. sometimes that magic is better than other times.