I'm, in the same boat, I replace as soon as they seem to have wear, and that's about one a yr for me
better safe than sorry

I seen my buddys reverse draw Horton explode while he was loading it, the server wear had worn into the main string, was 3 yrs old only and not shot a bunch too!
costed him about 500 bucks to fix, needed new limbs
was VERY lucky he didn't get hurt, or his kids, as he bought it for his kids more or less!
strings are cheap insurance , if they look worn, replace, better safe than sorry!
LOTS Of friction on them and a LOT of stress too!