I've noticed Izzy's teeth are pretty shallow now. I'm not sure if this is from fruit or just age, as she's getting up there now. She can't crunch hard/tough stuff now, for sure.
Age will wear down the teeth for sure. Not a whole lot to do about it once they are worn. Soft foods don't provide the abrasiveness that cleans teeth and gums the way shells of insects etc do. You can clean the teeth and gums using oral cleansers, but once the teeth are already worn down they can't be built back as far as I know. Same issue with human teeth in that regard.
There isn't really a set rule, it should just make up a small percentage of the overall diet. A few times a week sounds fine, but it depends on how much she is eating overall.