With all due respect to the poster who helped, that really isn't the best rule to use--as the OP says, that will not get "reset" if motion becomes active again. It will schedule the next action, the "off," every time the motion sensor becomes inactive. The revised "not pretty" rule above actually would work, though it's a bit of an unusual way to write it.
There is a basic motion lighting example in the Rule 4.0 docs that is exactly what you want. I would read those before you get too deep into RM--it would have saved you this trouble already. Second, if that's all you're doing in the rule, I wouldn't use RM at all. The built-in Motion Lighting app is a lot easier to set up and can handle both this situation and more complex ones with ease.