Any time from now.
Last year I was off to Australia early August (winter there) so I needed a new coat. Okay I WANTED a new coat. I was keeping a daily eye on things because of this new coat obsession.
From 25th ish June, the sales started. (I think you'll find John Lewis etc have started to reduce).
By about 4th July, many had started, or had hinted via various email alerts that they were going to start soon. Places like John Lewis, Debenhams etc are often quick off the mark, Next etc soon follow and smaller web places like Hush are last.
Obviously i was interested in New Season stuff which started to come in from 3rd week of July. (Not the full range but some coats started to appear!)
I left 8th August and several new waves of new stuff came out giving me about 5 days to make a decision and return the rejects.
By the time I left end of that first week in August, just about anywhere I had been looking had started their sales and most had some Autumn stock starting to come in.
Hope that helps!