Last year Mt. Holly and Pine Knob both closed on March 10. Alpine Valley was maybe a week earlier than that but honestly they were done by late February. Up north got killed last year, most resorts also done by March 20. In an average/decent year they can remain partially open until April 1. Boyne usually reopens for a weekend mid-April with a fun ski/golf combo pack for like $40 or something ridiculously cheap.
I don't think anyone's making snow anymore.
Even the places down here, usually have plenty of snow when they close, it's just a matter of people stop skiing and snowboarding, and start thinking about biking, boating, barbecuing, etc. Two years ago PK shut down with 100% of slopes open and 36" base everywhere. It was sad to drive past it and see how much snow was just untouched.