I don't think there is a specific set limit (maybe some schools). I think they like to see 20-30+ hours or maybe that you've shadowed in all possible areas of Optometry so you know what you are getting into. I have 30+ hours, but a lot of that is with one optometrist I go back to on a monthly basis to learn more about the field (not just the exams itself). I would just make sure you know WHY you want to go into Optometry and that you have exposed yourself to all areas private, commercial, surgical, etc.