There's no set time. Long enough so that he/she feels they know you well enough to write a great LOR. The longer the better. I shadowed a DO all the way up until matriculation, and I'll shadow him again when I go back for Christmas. To me, it's a great experience, and a good way to make possible future contacts, especially if you plan on practicing there in the future.
Shadow him/her for as long as you want or as long as he/she will allow you to. Or as long as it takes to get a good letter of recommendation. Or until you get bored. How do you know when to get out of the shower? When everything you set out to do is completed. Same rules apply. Good luck.
If you are haveing fun shadowing, don't stop, just keep on and on until med school. Maybe do a preceptorship later. Hell, I'd join the group of the guy I shadowed. Why, because I love it that damn much. REALLY!
