What difficulty did you manage with 6 hours of study?

Easy and medium. I wasn’t asked any hard questions in my interview.

While I would definitely say it's a good idea to study harder questions, my experience has been similar to yours in that I don't recall ever being asked a more difficult leetcode/HackerRank style question in an interview.

That being said, I've never interviewed at FAANG before. I suspect they are the ones who would be asking the tougher questions if only just to reduce the absurd number of applicants they receive.

I do have a PhD in CS (and even taught algorithms). I solved 500+ leetcode problems during 2 years, and failed a couple of time before finally getting a FAANG position. I'm sure that some people are smarter/faster than others, but in my case, studying was necessary.

Before preparing, I could solve most problems within a few hours. But it's far from enough. You're competing with people who are fresh out from college, solved 1000 problems, possibly who had leetcode classes! Some of my colleagues pretty much know solutions to all leetcode problem, just from the name of the problem.