I think that's going to differ widely from person to person.
And, I think you have to come to know your own body to educate yourself on how much you're pushing it.
Also, it depends greatly upon your goals--is it "just" weight loss" you're after? Or do you have other goals in mind?
For example, I ride my bike. Now--if I was just out to lose weight, I could ride for an hour or so a day and that would be plenty to accomplish weight loss. HOWEVER--I have other goals in mind then just weight loss.
--I want to ride in a 60 mile charity bike ride in October 08
--I want to ride a Century ride (that's 100 miles) by next spring if not sooner
--I have one more goal that I'm keeping to myself for now (sorry all but will tell share at some point

) that will involve a LONG ride over mutiple days.
Now, to accomplish the goals above, I have to ride more than 1 hour a day; so, I ride at least 30 miles when I go out and now do two 40 mile rides 2x a week as well. They take over 3 hours to complete.
Do you see what I mean about goals playing into how much you're exercising?
Hope I've helped!