To add a bit of information to the answers from Hilary and Charlie... In informal, spoken English, we would be more likely to use "many" in a negative sentence, but NOT in a positive sentence.. ("Many" in a positive sentence sounds VERY formal) We'll use "lots of" or "a lot of" in positive, negative sentences and questions. I have a lot of friends. I have lots of friends. I don't have many enemies. This is true for "much" as well, for uncountable nouns.

The things people say in conversation are usually not what you would encounter learning English. So, what I would "naturally" say in this context, particularly in speaking to someone, would be something along the lines of: "I spent tons and tons of time studying for the test." (a ton is a weight, a ton is a very heavy weight.) or, "I spent loads of hours studying for the test." (a load is an amount, a load is a large amount.)

Well, they are all "grammatically correct", but you would most usually hear/say "many". The other two alternatives might be used to insist on the length of time, e.g in an exchange like this: A - I guess you spent several hours preparing for the test. B - Oh yes, I spent a LOT of hours studying for it. (...putting the stress on the word "lot"... + the same for "lots")