much/many

In strictly correct English, (a) is the better choice because the sentence is a positive statement in which many is modifying a plural countable noun. According to this convention, much is used in positive statements with uncountable nouns:

There were many laughs.
There was much laughter.

These somewhat confusing conventions are outlined on this handy website.

That said, in less formal English (b) would probably strike a native speaker as fine.



Also, I would say that for is not strictly necessary but preferable. Grammatically, He studied three hours makes it sound as if the three hours were the object of his study rather than the duration during which he was studying. In practice, though, few native speakers would interpret it that way.

Both (a) and (b) strike this native speaker as wrong, not fine.

He studied for as long as three hours. Three hours is a period of time.

In fact, if you merely wish to state the length of time, he studied for three hours. If you wish to compare the length of time to some other shorter periods, you might write e.g. Some nights he studied for an hour, on other nights he studied for two hours, and occasionally he studied for as long as three hours.

True, but three hours is also a quantity of something--specifically, three of that something rather than two or four.

Quantities/periods of time always cause some difficulty when it comes to grammatical rules that depend on countability. Does one say "less than sixty minutes" or "fewer than sixty minutes"? In practice, both sound okay to me but I still don't know what the "official" ruling is on that one.

That's the official ruling, Shapeless.

The Oxford Dictionary seems clear enough:

...we say less than six weeks, not fewer than six weeks, because we are not referring to six individual weeks, but to a single period of time lasting six weeks.

... AskOxford

http://www.askoxford.com/asktheexperts/faq/aboutgrammar/lessfewer?view=uk