The following is what current Stanford students have told me:

Typically at Stanford the rule is that, if you’re looking for an A, you should assign 2 hours work for each contact hour. For a standard 4-year single degree, you’ll be taking 15 units per quarter, which means around 45 hours work a week. For a dual degree in that time, you’ll be taking 18.75 units per quarter (on average), which means around 56 hours work a week. Most students persuing dual degrees do it in 5 years, however.

For the single degree, that’s around 7.5 hours work a day, with one day a week off. For the dual degree, thats 9.4 hours work a day, with one day a week off.