So, basically, shit's fucked up. For many smaller/weaker weapons (daggers, straight swords, curved swords, spears, hammers, lances, and to some extent fist weapons) things work in a general rule of 1:3. For every 3 poise your enemy has, you need 1 poise-break to stagger them out.
How we tested it: one of us would be in a bow-drawn state and the other would attack until the state was cancelled. All references to r1 below are for 1handed attacks, 2handed attacks will be mentioned specifically.
How does poise work? It is definitely a value that fills up like a meter, and poise-break 'points stack'. Let's say you need 3 hits to break your poise? I can hit you 2 times, run around for a while, come back and hit you again and you would stagger. You can even stop your action then start again and that 'stack' would still remain. This is also why sometimes 2 consecutive hits will break poise and sometimes it takes 3 -- it's because there was a "roll over" point.
Maximum poise is 255 because From's team B is all about reliving the Y2K horror again, at 256 the poise rolls over into negatives. Currently it seems that Iron Flesh is required to achieve maximum poise as armor alone can't get there (even with ring of giants +2 and max natural poise).
So, an example. If you stack 150 poise -- a person with a mace would have to hit you 2 times with an r1 to break your poise. 2handing increases the poise-break, but I'm not sure if it's a doubling or a 1.5x. For instance, drake keeper sword needs 3 hits to break high poise (150+) but when 2handing it -- only 2 hits to break it.
Now, that's how things... usually work. With that 1:3 ratio... 1 poise break per every 3 poise. However, quite a few weapons just flat out don't fucking care.
All halberds that use the actual halberd moveset (including weird shit like drake-keeper warpick but EXCLUDING Syan's halberd) will break poise of any value with a single r1.
All greatswords, including the fucking keyblade, will always break poise no matter what. Greatswords are flat out broken; they feature retarded combos, stunlock, speed, AOE attacks, and now we can confirm that every single greatsword (bastard, masto, drangleic, etc. ) all break poise with a single r1.
All UGS and great hammers that actually have the proper moveset.
Curved great swords are busted; the Murakumo works as per the 1:3 rule... but the dragon curved sword is all kinds of stupid. In no small part, probably, because it uses the great-sword moveset. It follows the formula but 1handed r1 spam can also guarantee a 4-hit combo if you start close enough, or more as long as you have stamina and your weapon connects (it pushes the enemy back.)
Fist weapons will always break poise when doing a rolling attack, even a regular unarmed fist WITHOUT the vanquisher's seal.
I right now do not recall shit about axes, so... sorry about that. Axes are quite notorious for being weird tho -- in that their movesets are all the fuck over the place and they behave differently regardless of what their item description says as to whether it is an 'axe' or 'greataxe'.
Finally, stone ring seems to add 5 poise break.
tl;dr poise is absolutely busted and pointless, basically.
A couple of notes -- these numbers may change with the new patch and the DLC as I believe something about poise was mentioned. They would have to actually FIX poise tho, where-as I suspect the patch will just add poise to armor instead of actually making it work right.
From's designers are beyond sloppy and basically everything that is rule or system based is busted in this game.