[A certain length...Legosi has to stop and think on that for a moment.]

Well...that I think depends on the rodent. Some need longer teeth, like beavers...while others much less, like most mice I would assume.

[He rubs his cheek with a finger.]

But I think it all comes down to instinct. Your Pokemon should be able to sense when their teeth need grinding, and they wouldn't go so far as to hurt themselves. If that were happening, there might be a deeper problem...anxiety related, maybe.