[Edited: it seems we ninja'd each other.]
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 would not really be a common idiomatic expression in this situation; I'd be reluctant to use it in a context of frequent events, because I can't imagine a situation where you could not substitute often, a simpler word.
If you are not talking about something that happens frequently, but rather about a continuous length of time, you could use much of the time:
Much of the time we had left together
was spent looking for the right papers
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.