I have shadowed 5 optometrist, but none longer than 3 days. I ended up getting a letter from a guy that graduated in 2004 and I had only observed him for 4 hours. He volunteered to write a letter and he seemed like he would do a good job. I think it just depends on the vibe you get from the optometrist. I don't think it's hard to get a "good" letter of rec, but it might be hard to get an outstanding letter of rec. Hope this helps!