It doesn't say. But if you have 5 classes in high school, that would wind up to be a max of 10 hours of homework per night, a conclusion of about 15 hours of schooling per day. I would reckon that is not what is expected, so I read it as 'total'.
Surely it can't be per class. Depending on the class schedule of the school, that would be suggesting that 8 hours of homework for a high school student was "within acceptable parameters".
I think the better lesson is not, 'how many hours of homework is fair' but, 'teachers, please assign the minimum effective dose of homework'. This is one of the reasons I think labs are effective in high school science classes, provided they don't spell everything out for you and require you to understand what is going on, why you are doing it, and what's the important points are of what is going on. Applying knowledge is far more effective at making knowledge stick than balancing chemical equations.
K Occasional
1-2 15 minutes (M-Th)
3-5 30-60 minutes (M-Th)
6-8 30-90 minutes (M-Th)
9-12 60-120 minutes (M-S)