"much of the time" vs "most of the time"

I suspect the author in that sentence hesitated between often and most of the time: he wanted to indicate that it was helpful very, very often; but he didn't want to go so far as to say that it was helpful more than 50 % of the time, and so he chose a somewhat cowardly expression in between. I think he would have been better off choosing either often or most of the time, or perhaps very often, though I don't think the added intensity of very is really necessary.

Most of the time is an expression indicating that something happens more often than not, usually much more often.

Much of the time I would normally only use where you could not substitute often, a simpler word, which is the case especially when it is about a large chunk or chunks of a period, not merely a frequent number of times.

They arrived early at the aeroport. They had wanted to spend their final hours in romantic embrace, but they were busy looking for the right papers and documents much of the time.

This means that a large part of this time was spent looking for papers, but "large" could be anything from 1 % to 99 %; if I used most of the time, it would mean that more than half of the time was spent on it.

Some answers have claimed that much of the time is not an idiomatic phrase, and/or that it is restricted to particular meanings. However, Google claims around 171 million results for the phrase. While some of these are part of other phrases ("How much of the time", "so much of the time"), looking at the first few pages of results suggests that "much of the time" is a common idiomatic phrase, and that the contexts where it can be used don't differ significantly from "most of the time".

COCA shows a similar picture (494 occurrences, some of which as part of other phrases, but most not - though contrasted with 3655 occurrences of "most of the time"). So it exists, but is much less common than "most of the time".

My own experience, which seems to match the examples I've found from both sources, is that "most of the time", while technically correct for anything that occurs more than 50% of the time, tends to be used for things that occur much more than 50% of the time. The phrase "much of the time", then, conveys a sense of something that happens frequently, or for a significant proportion of the time, but not enough to call it "most of the time".

much describes the actions used within the time, as in "much of the time was spent washing dishes," whereas most is used to describe the person speaking i.e. "most of the time I like to wash dishes, but today I do not want to."

Of course, they're often interchangeable like most of the English language :-)