Our son went to a state grammar and they had their first "exam week" just before Christmas in year 7, so basically after 10 weeks or so of tuition! That was strict exam conditions, separated desks, etc.
But, I suppose he'd already done that by taking the 11+ which was truly the first "real" exams he'd taken. It's a long time ago, now, but that was 3 exams, all done in one morning, with only a short break between, so maybe 45-60 minutes for each exam?
From what I remember, that first "exam week" in year 7 was a full-on week of pretty short exams, usually an hour if I remember rightly as they did one before morning break, one after morning break and one after lunch, so 3 per day, 3 hours in total. I can't remember him being too stressed about it. The only vivid memory was his first test on one language subject where he did VERY badly on one section, scoring something like 1 out of 10, due to not memorising a grammar/sentence structure due to him not doing "rote learning" as part of his revision - he never made that mistake again so it "worked".
As the terms and years passed, it was just normal school life and by the time GCSEs came around, he was very blase about them, no sleepless nights, not particularly anxious, they were "just another school day" for him!
It's too long ago to remember my own school days (70s) with any clarity, but I do remember exam weeks back then where seemingly the whole school were doing them at the same time.