What I think of when determining the amount and type of reverb on anything is where I want that element to be in a 3D space and what I want that space to look like. Then I just go through a couple of reverbs until I find one that sounds close to that image, and tweak its settings and volume until I'm happy.
For example, if you want a snaredrum to sound in your face, you treat it like something that is actually in your face. What would that sound like? The direct dry signal would be very loud in comparison to the reverb, and the reverb itself might take a couple of milliseconds to reach you after you hear the direct sound, since it has to bounce off the walls and back to you. This would translate to a loud volume of the dry signal, a low volume of the reverb, and a rather long predelay on that reverb. The type of reverb you use would be the one that fits to the 3D space you had in mind (like a hall, a small room, a barrel, etc.).
Now for the opposite: if you want your snare to be part of an ambient landscape, you probably don't want it to stick out so much, so its direct dry volume would be medium to low. The volume of the reverb itself would depend on how close or far the snare is away from you in your landscape. Let's say that the snare is standing in the front of a church. When you are standing in the middle of that church, the reverb would be almost as loud as the snare itself, and the time it takes for the reverb to reach you after you hear the initial sound wouldn't be very long. So this translates to a volume difference between dry and reverb track of 60-40ish, a short predelay time, and a reverb-type like "Hall".
Now, if you would have been standing all the way in the back of that same church, the reverb would probably be as loud, if not louder than the original snaresound, and all of those sounds would reach you at roughly the same time. This could translate to a Dry-Reverb ratio of 40-60, with no predelay time at all.
Ofcourse these are just rough guidelines that may or may not work in your mix context, and sadly, there usually isn't a ton of room for reverb in a dense metalmix... but the only way to find out is to play around with it and learn. Also, don't be afraid of trying something that sounds UNnatural. It might just be the thing that makes an instrument stick out in a way that catches the listener's attention. Like the part you mentioned in mr. Sneap's mix. If that would have sounded normal, would you have noticed it?
Like others have said: there is no right or wrong amount of reverb, as long as it does what you want it to do.
