In terms of how long you should shadow an optometrist, it varies with different schools. Ohio State asked in the 2008 applicants to shadow at least a total of 10 hours in two different optometric settings. But yeah, almost all schools will ask you to have at least one recommendation from an optometrist, and usually just shadowing won't be enough for that optometrist to give a strong recommendation.
My situation was that I observed in my own optometrist's office the summer before my 4th year in college, for about 40 hours, so I received internship credit at my university. Then, I have decided to take a year off before optometry school, and am currently working at the optometrist's office fulltime. I'm also looking for opportunities to speak to other optometrists in different settings to get an idea of why they are doing what they are doing specifically in the field of optometry.