The term "blocking," simply means the process of "roughing in" the animation. Of course, because every shot is different, there is no "set number of frames" for the blocking of a shot. The frame count of a shot is usually predetermined by the Director and Editor of the project, or else the length of a piece of dialogue you might be working with.
However, the goal of your blocking should be two-fold:
1) Completeness (which, by the way, IS a word. I checked two dictionaries to be sure!)
Your blocking should be "complete," for lack of a better word. By this, I don't mean that the blocking should be complex or finished animation - not at all. Rather, I mean it should contain everything about your animation that you want to pitch to the director. Ideally, you should be showing all of your main poses, all of your main ideas, the full body language of your character, and all of the major timing decisions with as few poses as possible.
This likely includes all of your key poses and most, if not all, of your breakdowns, so that you are properly describing the weight and movements of the character, all of the acting decisions, and all of the ideas of the shot.
The other goal of blocking is to have something usable that you can continue to improve on. While it's important to show all of your ideas and actions in the blocking, it's equally as important to work "cleanly" and in a way that will let you easily edit your blocking once it's approved. You want to end up with a foundation that is strong and ready to be built upon.
As such, my recommendation is to avoid over-complicating your blocking whenever possible. Hopefully you have a supervisor or client who can understand what blocking is (and what it isn't) and you can show your rough blocking to them for buy-off before really fleshing out all the bells and whistles.
If you are working freelance with a client who just doesn't understand blocking, then my advice is to put together a little presentation for them to show the various stages of animation. It's good to have something around (from a previous gig or an animation test you've done) that you can use to show what previz (pre-visualization) is, what blocking is, what it looks like while it's being refined, and then what it looks like once it's polished. If you can get them used to this process early in the project, life will be a lot easier for you!
Best of luck! I hope this was helpful for someone...