I think we can do better than 11 characters!

I happen to have a corpus which includes pretty much every word ever written in a book, including many misspelled, mistranscribed, or otherwise non-dictionary words.

After eliminating nonsense, non-English, or other mistakes, I think the real winner, coming it at 12 characters, is:

    teetertotter

That's a relatively common word. Even though it's usually seen hyphenated, the unhyphenated form is recognized by all the online dictionaries I found.

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And some other candidates, just for fun, in the 13 or 12 character range:

    proproprietor
    priorityqueue
    reporterette
    preprototype

"proproprietor" seems more like a misspelling. Should have a hyphen, or be two words.

"priorityqueue" is of course familiar to hackers here, but is more of a jargon term, and is only concatenated due to appearing in source code. Invariably it's two words when actually written out.

"reporterette" is antique, but appeared in a NYTimes headline as late as 2018 - the author reflected on her career, including sexist epithets. https://www.nytimes.com/2018/12/02/opinion/george-hw-bush-ma...

"preprototype" is used exactly as is, in lots of scientific papers, up to the current day. That's a pretty good one too, and could be a tie for "teetertotter", but it's verging on jargon.