It's a bit tricky to calculate for Whistler, as the choice is between a season pass ($1,399 early bird price) and a 10 day EDGE card ($659) plus day tickets. It get's more complicated because you get free early season with the EDGE card (to December 15) and for $89 you get unlimited spring (March 24 to late May closing). Based on my distribution of ski days, the break-even is around 25-27 days. I didn't bother factoring in that some lift days could be $50 backcountry access lift tickets which would push the break-even out even further.
Add a third option - Mountain Collective Pass. For $379 that's 2 free days (at let's say $100 USD), then 50% off thereafter, so it's less than 6 days to recover the cost.
For next year, I've selected Whistler as my 3 day hill option on the new Mountain Collective plan, so it's barely 4 days to break even.
I was in late April and early May skiing for $31 / day using the half price system. Nice.