What Determines a Successful Parry?

Hi! I'm trying out the new Huntress in PoE2, and I'm wondering what the formula is for determining whether I can parry an attack? At the start of combat my first parry always lands, but then I'm not sure if there's a timer to when I can land another (and if there is, is there a visual indicator?), or do I need to do a certain amount of damage? or is it just random?

My understanding of Parry is (and keep in mind, this is from like, 5 minutes of trying it out on zombies) is if you are holding the shield up, any attacks are parried, unless you get Staggered. there is not a 'timing' aspect, and I don't think Red attacks can be parried (i.e anything that can't be blocked, can't be parried). Ranged attacks can't be parried outside of close range either (at least, as per the ability).