If you're looking for so-called "natural", I'd go with:
I'm answering from native speaking instinct on why "a lot of time" is more natural than "lots of time", or why "lots of gold" is more natural than "a lot of gold". This is just instinct; I cannot prove it.
But what I can say more definitively is that "a great deal of" and "a large amount of" are formal and "puffy" sounding. Nothing wrong with them, and of course people would write or say that...but when comparing these have a different character that might sound pretentious.
"You know that guy Bruce? He made lots of money selling ice to Eskimos, I dunno how he does it!"
That's "natural" and colloquial. If you start throwing in "a great deal" or "a large amount" you are starting to sound stuffy.
"Are you familiar with that fellow, Bruce? He made a great deal of money in retail; marketing ice to Eskimos. I am unsure how he is so successful with such a questionable venture, but he's managed to garner a profit!"
As for why "lots of time" doesn't sound as natural as "a lot of time"... or why "lots of gold" sounds more natural than "a lot of gold"... I can't precisely say. I think discussing gold discovery places itself at a certain historical moment, at which you might want to slip into the casual speech of a gold prospector...whereas talking about spending "a lot of time" is something we say day to day, and has a little stricter rules?
UPDATE: I hadn't noticed the periods here (or not considered them). As pointed out in the other answer, if your sentences really are this short instead of having additional information, they are all awkward because they feel like incomplete thoughts...especially the statements about time. You'd expect the sentence to tell you what the person spent a lot of time doing, such as
He spent a lot of time working in the garden. The rest of what I said would still apply.