How much of a difference in gravity would there have to be before it become problomatic for humans. .2G, .5G, 1G, 2G?
Depends on the human. As I said, think of it as carrying a weight equivalent to the added percentage of your own body weight -- or, more accurately, like having weights strapped to your arms and legs as well as on your back. The stronger and more physically fit you are, the better you'd be able to handle it.
Although having to carry that extra weight all the time would put more strain on the body, and on the heart. Again, the better your fitness, the longer you could stand it. (Although just living in high gravity would pretty much compel you to get into better shape.)