Time and work-load aren't necessarily correlated. When my ds was in elementary school, he spent an hour on his spelling homework of writing the words three times each. It took so very long because it was so very boring, he couldn't stay on task.
It was only after years of homeschooling, and then him finding his own motivation to get things done in a more timely manner, that he actually has (re)learned to focus. This will be the first year he will be attending school (just math and science) in a long time, so I am wondering how it will turn out. I know someone who has a son at the same program, and from what she said, her son sounds PG; he spends 20-30 minutes a day on that homework, but she had been asked about it by the school, since so many parents were complaining that their child was spending 2-3 hours a night on it. So in that case, it depends on the ability of the child, too.
I personally think our country has gone homework crazy -- "See what a great school we are, we give the kids X number of hours of homework a day!" If they were doing a great job, the kids wouldn't have to learn the material at home....
