Longest kanji reading

What's the longest word that can be written with a single kanji?

Chinese limits itself to a single syllable per character, but Japanese has no such limitation.

One, two and three kana-readings are fairly common.
There are not many characters that have four-kana readings, not including any okurigana word endings.
紫　 むらさき murasaki [purple]
志す　こころざ・す kokoroza-su [to aim to be]
快い　こころよ・い kokoroyo-i [agreeable, pleasant]
There are only two five-kana readings in the Joyo kanji set.
志　こころざし kokorozashi [will, aspiration]
承る　うけたまわ・る uketamawa-ru [to receive, undertake (humble)]

Outside the joyo kanji, and you find that people have got pretty creative with kanji throughout the years. As you can create new characters with new meanings, and as Japanese permits multiple syllables, one character can represent a whole sentence.

One of the longest 'accepted' readings is for the character 砅 もをかかげてみずをわたる mo o kakagete mizu o wataru or いしをふんでみずをわたる ishi o funde mizu o wataru
Which means: crossing water by stepping on stones.
That's a 12 kana reading.

It's a bit of a cheat, really, because it's more descriptive than an actual word, and the 'reading' can be written 藻を掲げて水を渡る which is easier to understand. In fact, the single character is only rolled out as an example of a very long reading.

The character also has the distinction of actually existing in the Unicode character set. Most characters with very long readings only exist in lists of characters with very long readings.

However, the candidate for the longest reading in the JIS character set, at 13 kana long is:
砉　ほねとかわとがはなれるおと hone to kawa to ga hanareru oto
'The sound of skin and bones separating'
Delightful!

Coincidentally, both 砅 and 砉 appear on the same page in my Kanji dictionary (漢辞海). Somewhat disappointingly, it doesn't list these long 'readings', but it shows that when you get to the outer fringes of kanji, pretty much anything goes.

More here (Japanese):
http://www.akatsukinishisu.net/kanji/nagakun/