That's a very tricky question. I think Driver answered it pretty well, but I will never forget when one of my friends at PSA was about to upgrade and told me on the phone that he had never had to deice and was worried about being the one in charge when that time came. I think as far as acting PIC in a 121 environment goes, it should at least take 2 full seasons in all kinds of weather conditions to be considered Experienced. (I say this because when I was hired I had no icing or thunderstorm experience myself!)