WTG normally makes it to mid March for sure. Some years they have stretched to early April and had a park party to close the season, where they scrape up excess snow and make tons of huge jumps.
Last year their last day was March 23 (happened to be Easter Sunday -- I skied there, and it was a really great day on the slopes). Normally they back down to only 2-4 trails at the very end of the season. Eagle's Swoop and Dobie are the first trails to open and often the last to close.