In my opinion, shadowing is the most important part of what you do as a pre-optometry student. Not only for your applications, but for yourself. I shadowed for an entire summer. Yes, I gave up a paying job that summer and others may cite this as a waste of time, but there were several advantages:
1) I got an AMAZING letter of recommendation from the ODs,
2) I got a job offer for when I complete my degree,
3) I built a close relationship with two ODs who are more than willing to give me guidance if needed,
4) when I was asked hypothetical questions about how I would practice during my interviews, I was able to bring it back to my shadowing experience and fully answer the questions using my OWN experience, not just what I assumed it would be like
5) I KNOW this is what I want to do, without a doubt.
Don't skimp on the shadowing just because there isn't a specific minimum. They DO weed people out who don't seem to know what they're getting themselves into. Even if you don't think that shadowing is crucial, spend a little bit of time doing it, then think about getting a job at an office or something else that would give you first hand experience. After all, would you really want to spend 4 years and a LOT of money to find out that it's not what you want to do when it's WAY down the line? Find out now.