You may be able to raise them all summer long. Just be prepared for a way to cool those birds off if you get some temp spikes like the PNW had last summer. I'm in the WA cascades at 2200' and we had 10 days of 105-110 last year. I had Cornish X's at the time but they were still in the brooder. They were 3.5 weeeks old when the heat wave hit so I just kept them inside where it was cooler (Earth berm basement) until it passed. I think I would have to run out and get a little mister setup if they had already been outside.
I plan to get another run of Cornish crosses at the beginning of July. I could possibly run into some terrible days of heat so I'll have a mister setup ready to go. If it gets too hot, I'll just move the tractor within hose range of the spigot and run the misters on a timer or something until the heat passes.
So far this year, we've only had one day where we barely reached 80° and that was just for a few hours. Today it was 50° and they've been calling it "June-uary" here. LOL
I am so sorry I didn't even notice you were in Idaho! This was my first year with CX so I only have tis experience to go on and as you know the weather has been weird here. I butchered all of mine this morning woo hoo the people in Fruitland are GREAT 25 birds vacum sealed and in the freezer.
I can say this though I was planning to get another 25 however am going to hold off this year I don't want to attempt this in the late July and August heat. Next year I will start earlier and do two batches.
Where are you located? I'm in Middleton . . . there are a few others on here in this area; and if you are looking for chicks, John in Payette is selling a new batch of CX that just hatched, I got mine from him and they were great.