I'm not exactly sure, I was on bars a ton and so I had to get new grips quite a bit. I knew it was time when they were so thin it was as they were about to rip. They did tear a little on the sides though and that is when I ordered them.
How long have you had these grips?
It really depends but you might want to have a back up pair broken in just in case. My friend was at a meet and while she was warming up her grip somehow tore in half but she had a back up pair so she was ok! =D
Ahhh if your grips are tearing on the sides, then get a new pair ASAP. I've had teammates whose grips totally tore in half while they were on bars.
My first pair of grips lasted for 2 years. By the end of club (L9), I was going through two pairs of grips per year. I like thicker leather, though, and during the summer, we were on bars 1.5 hours every day, plus 3 hours of bar classes every week. Needless to say, my bars drastically improved the last two years in high school after I switched to that gym!
If they are starting to tear, get a backup pair broken in soon! You never know when they're going to break. But as long as you feel safe in them, I suppose you can keep using them. If you've got a lot of small tears already, though, you might want to retire this pair so you don't have them fully rip on you during the middle of a skill. I like to "rough up" my grips and after a while they get so flattened that I can't rough them up anymore and that's when I get new grips. Most people keep their grips longer than that, though.