Hey Yazo!
I don't know that every school has an exact requirement. The purpose is really just to get exposure so you know what the field is like. For your own sake. You wouldn't want to blindly go into a field you knew nothing about. Through shadowing you may realize one of two things. One, "This is not for me." Or two, "Wow I really see myself here in optometry." I agree with SunA21, I think it also looks really good when you shadow different optometry settings. There's a whole bunch like private, group, commercial, military base, OD + MD partnered office, vision therapy. Enough hours is when you've gotten a good feel for optometry and/or quite possibly enough to ask a doctor to write up a nice LOR for you. And of course, you will be able to write a colourful statement and deliver an eloquent interview with your knowledge of optometry gained. Lots of bonuses to shadowing. Way to be ahead of the game! Cheers!