As far as shadowing goes, I think you would need enough to show that you know what you're getting yourself into. I personally shadowed for close to a year for about 2 hours at a time, about every other week. I found that this helped to keep in my mind why I was so excited about medicine in the first place (the daily grind was getting me down).
As far as a journal- I never kept one, but there have been a number of incidents that stand out in my mind as especially memorable.
Good luck🙂