Most of the time is an expression i

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: