how much prep for GRE subject exams?

How many hours of study is necessary for these?

I know it's hard to say, it depends on the person, etc, but I would like to get some rough ideas.

To simplify, imagine the person knows nothing about the subject matter. Imagine they are smart (let's say they've done well on another GRE or similar tests), learn fast, are motivated, etc. Average memory.

I've seen many people post about difficulty of GRE subject exams, but usually it's hard to tell how many hours they put in, they say how many months they studied but not how much time per month.

My guess would be "however long it takes to know the material in 2 test prep (review) books & read 1-3 textbooks" - agree? Along with doing some practice tests. Which could be 40-100 hours total to get a decent score?

(I feel like just one read through of textbooks would help? Mostly relying on the summary prep books and memorizing the key things in the textbooks.)

Your comments are appreciated