70 - 40 seconds Cooldown (based on level). Guardian only goes on cooldown when the shield is triggered.
Guardian Is Overperforming After Its Recent Buffs To Consistency. Bringing The Base Shield Value Down Back To Before It Was Buffed.
SHIELD : [80-200] ⇒ 70-150
Guardian is a powerful keystone that’s been largely unpicked and ignored, despite outperforming other keystones like Aery and Aftershock on individual champions. We’re zhuzzing it up by giving players more agency over when it triggers and sharpening its intended defensive output by replacing the movement speed buff with extra strength in the shield. Altogether, we’re aiming for this keystone to still have a similar power level despite these changes, but finessed in a way that is less frustrating and more reliable to play around with. Maybe it’s time to let your guard down with this one!
TETHER RANGE :  ⇒ 350
TRIGGER CONDITION : [Any damage] ⇒ 90-250 postmitigation damage in the last 2.5 seconds or lethal damage
| SHIELD AMOUNT :
MOVEMENT SPEED [20%] ⇒ No longer gives movement speed
Cooldown increased at early levels, decreased at late levels. Shield base value and ratios increased.
Guardian is the newest in "hidden power" mechanics: Like Bond of Stone or Aegis of the Legion's old aura, players are getting a lot of power from it, but they often don't feel it. We want to make Guardian a little weaker (it's overperforming), but also more noticeable (it's underappreciated).
COOLDOWN :  ⇒ 70-40 (at level 1-18)
BASE SHIELD : [60-150] ⇒ 70-150 (at level 1-18)
SHIELD AP RATIO : [0.20] ⇒ 0.25 ability power
SHIELD HEALTH RATIO : [10%] ⇒ 12% bonus health
DEFINITELY NOT BRAUM Guard allies within 175 units of you and allies you target with spells for 2.5 seconds. While Guarding, if you or the ally take damage, both of you gain a shield and activate haste for 1.5 seconds.
COOLDOWN 45 seconds
SHIELD 60 - 150 +(0.20 AP) + (+10% bonus health)
HASTE +20% Movement Speed