Snowboarder since age 12 (37 now). They both take about the same time to learn. I've seen some people start carving on their first day. Snowboarding is physically more demanding because in order to maintain stability, you need to have your weight on either the balls of your feet or on your heels. This means that at any given moment, your calves or your thighs will be activated. Unlike skiing, if you try to distribute your weight evenly between the balls of your feet and your heels, you will "catch an edge" and fall over. This biomechanical fact is most evident on shallow sloping runs that curve either to the left or right. Depending on whether you ride goofy or regular, on runs like that, you will have to spend a long time with your calves or thighs flexed in order to stay upright and that will get tiring quickly.