At the mountain I work at (which will remain unamed), there is absolutely zero guarantee that you get 30-40 hours per week. Full time instructors get a minimum of 4 hours, part time gets 2 for days that you are scheduled. Generally you get 5-6 hours of work the days you are on schedule, that being said, this season has been much less given the weather. I know most instructors this season are making far less than they did last season.
I work at grouse as a ski instructor. at the beginning of the season you check off what days you're available for the season and most of the time you will get scheduled for those days. you can work only mondays if you want. or you can work 7 days a week, but once you mark off you're days; they're pretty much solid.