You'll be working for Vail which means as a full time instructor - 5 days a week, roughly 6 hours a day (depending on whether you're kids/adults, this changes). As an instructor, you can occasionally backline and not work on slow days, but as a full time new instructor, everyone else will get first opportunity at this before you because you won't have built up priority yet.
The good news is - your days off will most likely not be on weekends because that's the busiest time for ski instructing. Also, Park City has night skiing, so there's that.
My recommendation is if you can pull it off and pay the bills, go part time. You essentially make your own schedule outside of working all the holidays which means more skiing, less work. Also means less money, but if you want money, being a ski instructor for Vail isn't the way.
I work for Keystone - so I don't know the specifics on Park City, but from what I've heard from other instructors, all the Vail resorts are run pretty similarly.