Quicksilver's ability is limit once per phase. The villain phase is separate from the player phase, so you can (and should) exhaust Quicksilver to defend on every Villain attack you can, since he will instantly ready after doing so, and this will NOT prevent you from readying on your next turn.
If going all in on the Protection aspect, you can even exhaust to defend an attack that targets you, ready Quicksilver due to his Super Speed ability, and then Exhaust to defend an attack targeting another hero. You won't instantly ready after the second attack, but if you have Always Be Running that may not be a huge issue.
Quicksilver combos very well with a teammate running Leadership, so you can stack up Attack and Thwart buffs like Lead from the Front and Morale Boost on a turn you have several copies of Always Be Running and smash the villain or main scheme with multiple huge hits.