How long does Slopes Premium stay unlocked?
As long as you recorded with an active paid pass or subscription, those recordings get Slopes Premium forever. Oh, and as a bonus while you have an active subscription any activities you didn't have an active pass for in the past get unlocked for free.
Note that this doesn't apply to recordings that were recorded while on the free trial if you don't upgrade to a paid subscription when the free trial ends. Once the free trial ends, those recordings will lose Premium.