The point at which folks change them seems to be mostly about how well they play. Cotton strings will often break long before the string is really worn out, but poly blends and similar materials should rarely if ever break. I find that they is a short break in on most strings, maybe when I see as about 15 or 20 minutes of solid play, and then they last for a bit before losing their bounce. I find that most strings will still play well and feel nice even after losing their bounce, but then comes a point where the string just doesn’t respond well. The string tends to look thinner/stretched, binds become inconsistent, it will start to feel stiff, the yoyo will really thunk at the end of the string in a different way, and tension becomes much more difficult to adjust and manage. For me, this is pretty obvious and the point where I change strings. It means I get around 5 days, playing a couple of hours a day, out of cheap strings, and often several weeks out of something boutique like YYSL Type X.
YYSL Type X is a great string, but it’s one of the LEAST durable of the boutique strings. I don’t get much more (if anything) out of a Type X than I do out of a Kitty. I suspect when we’ve paid that much for a string, we play it longer out of obligation to recover our investment. 
I use regular Kitty and I change it just about every hour of play. Or when I notice it doesn’t whip anymore without getting tangled really easily. And I notice Kitty tends to start to fray after about 30-45 minutes of casual play.
I change string when ever and for what ever reason I want. I do it automatically. Sometimes I simply want a different color, or just the feel of a new string, a different thickness or type, not satisfied with how the string is playing, or maybe I’m bored. The most important thing to me is simply to change it before it breaks.