I've only published 2 multi-chapter/episode stories but from what I think I've learned from that, and listening to other writers, it may depend on how fast you're writing and how often you want to post new episodes. And perhaps, too, whether you know exactly where the story is going - are you halfway or almost done or could you suddenly get stuck, unable to decide how to go forward.
So, I guess what I'm getting at is that it may really be a question not words but of the number of chapters/episodes you can smoothly divide the 60k of words into. How much time will those chapters buy you so that the posting schedule doesn't catch you with nothing ready to post?