Whatever is the lowest you can do and still feel like you are having fun I guess. But the only way to really clear one is on one credit. If you use a continue nobody will even bother recording your hiscore. Once you start playing them to clear them they quickly get to the point where they lose all satisfaction after you've continued. It sounds like you are right about at the line where you get to that point. And yeah they require a stupid amount of time and don't fit into a busy life very well.