ok the condition of my string is fuzzy and seems worn but is not going to break when i throw and it does seem thinner then when you get a new string. It also does not bind as easy i’m wondering what condition is your string when you decide to replace it?
Well, really whenever you want. When you think the playability is dropping and noticeable, change it. When you think the string is about to break, change it. When it has too many knots or is sticky, CHANGE IT. Don’t worry about it. You will know when to change it. Change it when you want. I would change it in the condition you were describing. So that’s really all. Hope this helps. 
I change mine as soon as it starts to fuzz.
I change mine when the colors get fuzzy. Not when the get physically fuzzy, but when the colors start to blur. I use Chaos 422’s, so when that starts to happen, I figure its to the point where the strongest fibers are starting to fray. Other that those, I just use strings till the life goes out of them, and they lose their bounciness. and their condition starts to jack up every other trick.
perfect thanks guys
Oddly enough, I almost never change my string. I yo-yo almost strictly 5a, and I change the string once every 2 weeks to once every month, yo-yoing for 1-3 hours every day. So when you change your string really is up to you. If you feel like it’s affecting your play, just change it. It can’t really hurt. Unless it’s like a spikey string or something.