I never actually "Documented" my shadowing hours.. But, if I had to I have a way to contact the individual that set me up with the shadowing experience. This way, if I ever had to provide proof, I would ask them to write up a short email or something.
As for how much shadowing you want to do is entirely up to you. I did 30 hours because that was the number of hours the pharmacist was working that one week. I did one full week of shadowing.
If you did need to document the hours, I would simply have a document that the pharmacist could sign off on stating the days/hours you shadowed. Also, make sure to get an email or phone number of the pharmacist in case you ever have to contact him/her.