That is hard to define...but you will know it when you see it if you can look at other turtles/tortoises of the same species. You need to know what "normal" is. It should not interfere with feeding, and of course will if seriously overgrown. Examine your husbandry and see if there is something you can do to prevent overgrowing. Some think diet contributes to overgrowing if they only get soft, easy to eat items. Others believe some abrasion by putting food on flat rocks helps keep it in trim. I have seen it in rehab animals fed a diet of lettuce...less common in animals fed a varied diet that requires them to work a bit, i.e. cactus, cuttle bone, etc. Good luck, any(?) competent VET can trim it, and you can learn yourself by watching if you are the confident type. You must know what is normal so you/Vet can grind to an effective cutting tool. Good luck