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In the age of abundance, the apparent availability of virtually all material necessities tended to lead people to expect speedy gratification of their desires and to have little sense of the length of time over which people in other times and places had had to wait in order to have some of their more basic material needs satisfied.

About 'over which', if you were supposed to pin down the single word that the 'over which' refers to, which word would it be; 'the length (of time)' or 'time'?
Comments  
is both:
the respective time interval
So you don't think we can pin down a single word, right?
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no,
a length of time
is a group of words with a meaning different from the components

but if you want to simplify things
it would be
time
which is more important
Marius Hancubut if you want to simplify things
it would be
time
which is more important

I don't think we can simplfy it that way, because 'people had to wait over time' wouldn't really make sense, would it?

But I don't think 'people had to wait over the length' would make sense either, so I asked the question.

It should be something like 'people had to wait over a certain period(length) of time'.

This might be the proof that things are not as easy as what the grammar books say.

Oh, by the way, I've seen you recently helping a lot of people, and you look good at English.

Just out of curiosity, what is your nationality, may I ask?
I meant to simplify things for understanding, not to change the phrase
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Right.

It's just, I didn't think it would be so easy for my students to understand it because they usually learn the mechanism of relative pronouns as something like this:

That is the house the house ten years ago=>That is the house in which/where I lived ten years ago.

But, as I said, it didn't seem possible to apply such mechanism directly to the sentence I posted first.

Well, and what about the other 'question' that I asked?