Synchronicity is an interesting concept... I've been thinking about exactly how to express my take on this topic, when Wayne Muromoto put this on his blog today:
http://classicbudoka.wordpress.com/ (Kaizen Budo).
Does the learning ever stop? Sure, if it's as far as you're comfortable going. Does the potential for learning ever stop? That depends on how you're classing "learning" here. I'll explain.
A theme I've seen here, with a couple of exceptions such as Flying Crane's comments, seems centered on "learning" involving new techniques or ideas. MJS was advised that he should attend seminars as he wouldn't see anything "so new that (he'd) go ooh aah over it". There has also been reference to "learning everything an instructor could teach", which seemed to me to imply a number of techniques. Now, in that regard, there certainly is a limit to what you can "learn" without looking at other arts. But to me, that's really missing the point of "learning" in martial arts in the first place.
To me, "learning" in martial arts should be better described as "developing". If we were to look at an art like Seitei Iaido, there are 12 techniques. That's it. 12. And they're all done solo, being relatively short sequences of movements. There aren't really variations on them, either. Now, realistically, a Seitei Iaido teacher could "teach" me all 12 in one afternoon without any real problem... hell, they could probably do it in two hours or less, including practice time to make sure I have them down to some degree. So why would I keep going back to that instructor after I had "learned" the 12 kata? Well, to continue to develop and improve. New techniques aren't the point. And if you're moving on because you're not being taught new techniques, I'd personally ask how well you've actually learnt the ones you have.
Now, this isn't to say that you should stick with the instructor no matter what, it's saying you stay with them as long as they keep you developing and improving, and as long as you want to keep developing and improving in that art. If you can do the techniques, and your instructor starts asking you for guidance and insight, then you've outgrown them. If you're doing the same techniques, and they're still correcting you, you haven't. Whether it's new techniques or not.
So to get back to it, does the learning ever stop? Yeah, it can. But that's more to do with the student than the instructor in most cases. And it's certainly more to do with the student than the art in all cases (excepting the more laughable ones... Ashida Kim, I'm looking at you!).