Candide Thovex shows that even dirt is skiable, so... year round!
For snow that might make it worth it to actually hump in skis and boots and ski some continual lines in the Sisters, I would say end of June? Mt Hood and Jeff > likelihood of skiable lines after end of June? But even so tend to be pretty bare after mid-summer.
It just had a dusting of snow up there 3 days back! So like, now! They are saying that the snow will be melting faster than normal this year as it will be a warm year. So ...NOW. Next month (July) it's gonna be badly sun cupped and beat to crap is my guess, a month earlier than normal.