I didn't even see this until now!
Hey Ash.
I shadow routinely, both a staff pharmacist (borrrriiiiiing) and a clinical pharmacist (exciiiitttiiiiing) on a semi-weekly basis. Next week I'm shadowing her for warfarin clinic, that should be a lot of fun!
There is no "set" shadow time, as you can gather by the responses you've gotten thus far. I have a really entrenched position as a volunteer in my hospital, and I have managed to shadow pretty much every clin.pharm in the hospital that would have me. But, other volunteers only went once or twice for 2-4 hours or so. It really is personal preference, how well you interface with the pharmacists, etc, that will determine how much, if it all, you shadow.
What do you do whenyou shadow? Well, it really isn't calling shadowing for nothing

Basically the drill is, you don't exist. The pharmacist will probably introduce you if applicable as a pharmacy volunteer or whatever, and from there on out, you're silent as a mouse, a fly on the wall, or, if you will, the pharmacist's
shadow. You don't make faces, you don't go "sssss" under your breath... nothing. You stand there and learn.
Might sound boring, but you can learn a LOT, and the experience will pay dividends in spades when it comes interview time and maybe even handy with LoRs/application!