It is a flat 20%, however there is a 3 second hidden cooldown. This means that Windfury cannot proc more than every three seconds, and any potential procs in that window are wasted. For this reason it is always a good idea to get a slow weapon so as not to waste any potential procs. Generally 2.6 speed is accepted as a minimum.
2.6+ speed is easy to come by in 2 handed weapons, but it can sometimes take a little effort to find a good slow one hander.
Hope that helps