Let's say holding the block button for longer than two seconds will activate the bashing animation... what's stopping the player from holding the block button for two seconds, releasing the button momentarily, then holding the button for two seconds again?
Unless fatigue drops whilst blocking (which makes NO sense by the way, if that's the case, then realistically (which is clearly the design of mechanic of Skyrim) walking, running, jumping, swimming, mining, smithing, farming, etc will also need to drain fatigue) it won't work.