This one's not totally clear to me. Let me read it out loud again. "Where's the dividing line between the craving for the end of Dukkha and serious practice?" OK, it's not really clear to me, because there's no real clear dividing line between these two. So, the question itself is not right, there really isn't a clear dividing line.
But we've got to be careful that we don't just sit on the craving for the end of Dukkha, and that's all we do. So the craving itself is a wise desire, it's based in Compassion for us, hopefully Compassion for others. It's a wise desire. It still interferes with the practice though, if all we do is treat it as a desire. If we sit down and meditate and all we're doing is, "Oh, I don't want all this Dukkha, I don't want it, I don't want it, pain, I don't even want this retreat, I don't want anything," We don't go anywhere! The craving itself can be a motivating factor, but if that's all you do it's not going to be good enough. You want to use it as part of your Right Thought. That you'd like to end Dukkha, you'd like to end all of it, out of Compassion. You've got to say to yourself "OK, I've got to work towards this end." You can't just sit there and wish it, you've got to work towards having the wish fulfilled. So there's no real dividing line, it's just getting the right balance.