So, following up on my previous post, which in comments developed into a discussion of Sino-Japanese "repeat previous character/s" marks, I found the mother of all "long wave" characters in an episode of Crayon Shin-chan (『クレヨンしんちゃん』, Kureyon Shinchan, a long-running gag manga (and anime) about a little boy called NOHARA "Shin-chan" Shinnosuke). Check it out:

The rightmost speech balloon is a train announcement saying that they've reached Action Zoo (don't ask). What's actually written there is "アクション動物園前　／＼", which expands to "アクション動物園前　アクション動物園前": "Akushon Doubutsuen-mae, Akushon Doubutsuen-mae." The "long wave" here is repeating nine characters corresponding to twelve morae.

Well, I thought it was impressive.

Incidentally, Crayon Shin-chan is written and drawn by USUI Yoshito (臼井義人) and I love it, non-ironically. It makes me laugh out loud constantly. And his mother and teacher are total foxes.