I agree that this is a subjective thing.

I always encourage people to move on when they feel comfortable with the material. By that I mean that you grasp the content, and you are aware of what you are trying to do with it. Perhaps you can't play all those chords perfectly, but you know how to practice them and are aware which ones need more work then others. The concept is there, you just need to better the execution.

It's good to move on to the next lesson, but keep going back to that old material as part of your routine. I agree that it's no fun to simply play through those same lessons over and over, but try after practicing the next lesson going back at the end and refreshing yourself on the older material.

Oh yeah! And the most important part. Have fun with it :D