The PISA results in Western European countries must have some comparative relevance though? Not in relation to holidays, I agree.
We are in our second week of summer hols here. DS (9 yo) goes back mid August. It totally irks me as it's mind numbingly hot and it doesn't feel like a fresh start as i'm not buying new shorts and tee shirts shoes etc for four to six weeks until the weather changes. 
Although Hungary and Poland have long summer holidays, my colleagues tell me that the A/W term has hardly any decent breaks. DS doesn't have the 5 on 1 off, either. It depends on religious holidays. If Easter is early summer term can be 14 weeks long without a decent break. He was knackered when that occurred.
His school hours are only 8-12 every day, with an extra 2 hours in the afternoon on Tues & Thurs, but as there are only 9 children in class (rule is a maximum of 16 per class here) the work is quite intensive as the teacher and teaching assistant are constantly driving the kids. He usually has an hour of homework to do on his "off" afternoons.