You know, ultimately this is all relative to how ready you are to begin with, and what you're shooting for(just pass, or max out?).
If you think taking the SAT now would be a friggin joke, then you can probably skip any studying on the math and verbal, as the ASTB is the same, if not easier.
Similar points apply to physics(the mechanical section), the vast majority is mechanics and basic electrical, but they might throw in a curveball like optics in a question or two. Also expect a lot of "gears" and "pulley" stuff which isn't covered in your typical physics course, but is pretty easy to figure out, and is covered by the Marine gouge.
If you read up on aviation/naval history for "fun", you're covered for roughly half, if not more of the aviation/nautical portion. Then there's stuff you probably learned already while getting a PPL, boating license, or meterology degree.
If like me, none of those apply, you're going to have to do your own dirty work and research that stuff.
For the spatial, I recommend figuring out a system...I used heading, pitch, horizon and tested it on the Arco as a confidence booster.
The Marine gouge is good and basically helps you study for the stuff you can probably expect on every form, but IMO getting that stuff down cold won't guarantee that you'll ace it. It's really up to you to figure out what you want and how hard you want to work to get there.
The Marine gouge will probably pass you, but won't guarantee you'll ace it.
My $0.02 on the subject.