Weekly...well, more every 4-5 days.
Our extreme weather waves rarely last more than a week.
I believe @ChickenCanoe recommends weekly where long term extreme heat happens.
I stopped with ice water(saw a bird gorge on the ice water and stagger around with an ice cream headache-seriously, I thought she was gonna keel over) so just the big chunks of ice now.
IMO, no reason ever for ACV...or any probiotics.
Most chicken feeds already have 'probiotics' in them(read...
Probably, but not many folks are able to access a good vet as you are.
I have done this in the winter, bring them someplace slightly warmer(not too much of difference-can cause shock going wither way) just for a couple-few hours, and give them a dose of electrolytes. Called the temperature...