1. Many have been surprised at the length of time it has taken him to make up his mind.

他居然花了那么长时间才拿定主意，许多人都对此感到吃惊。

2. He was disgusted at the length of time he had to wait.

他非常讨厌要等那么长时间。

我的答案是：两个定语从句的先行词不一样。

第一句先行词是time，作taken 的宾语，关系代词是that。道理很简单，“花费”的宾语是时间本身，不是 the length。

第二句先行词是 the length of time，作 wait 的时间状语，关系副词是 during which。道理很简单，wait 的状语是个时间段，不是 time 本身。

1. Many have been surprised at the length of time (that) it has taken him to make up his mind.

2. He was disgusted at the length of time (during which) he had to wait.