"It is all the more shocking to know that the hearing was less than half a day long."
the hearing （主）was （系）less than half a day long（表）.
less than half a day ，名词短语，副词用法，作程度状语，修饰形容词 long。形容词 long 是表语中心词，没有它，那么“听证会”与 “时间” less than half a day， 无法形成系表关系。

我再给你几个类比句子：
The man is twenty years old.
The street is ten meters wide.
The meeting is three hours long.

形容词是表语的中心词，不可省去。但之前做程度状语的名词短语，可以省略，句子照常有意义。