This is one of only three everyday kanji with a 5 morae reading, and one of only two nouns, namely 志(kokorozashi) (nominalization of form of verb 志す(kokorozasu), which has a four-mora reading). The only other kanji with five-mora readings are 承, in the verb 承る(uketamawaru), and 詔(mikotonori). No kanji in common use has a six- or more mora reading.
The unusually long reading is due to two factors: 志 is a nominalization of the verb 志す, which has a long reading (4 morae, kokoroza-su), which itself is a noun-verb compound, 心指す(kokorozasu). The nominalization removing the okurigana, hence increasing the reading by one morae, yielding 4+1=5, or more finely 3+(1+1)=5 (compare common 話(hanashi) 2+1=3, from 話す(hanasu)).