Most commerce classes (like the real subjects, not management or marketing crap) expect about 10 hours a week per subject work of you including attending lectures and tutorials. 2 hr lecture + 1 hr tutorial is generally the way it goes. The 2 hr lecture can be split up into two 1hr lectures. 7 hours should be spent studying generally but it's just a guide. It depends what mark you want. Also that's not including assignments.
As for law, it requires a lot of reading and such, it's not easy and takes a lot of work; I'm guessing 10+ hrs as well per law subject if you want to get HD's. The 10 hour commerce guideline per subject probably takes account decent students who want to do better than just "P's get degrees" or really dumb people who need to spend 10+ hours regurgitating just to pass.
It all depends if you're willing to put in the work