For me, how fast I pick it up is correlated to the number of hours worked per week. I became an almost-expert at Kroger because I was consistently working 20-25 hours or more per week (this was during prepharm). Now in pharmacy school I work intermittantly at best at Wags, and I don't know crap about it. I can't pick it up either, because I'm not there enough to practice and reinforce what I learn.