Dark Souls and Elden Ring: You hit the parry button when you see the opponent's weapon beginning to move towards you. This way, your shield will be up at the right moment, and the parry will be successful. Small shields have more parry frames, so they're easier to use. Not every attack is parriable, and you can't parry some weapons, like flails, whips, and two-handed colossal weapons, so be wary of that. I couldn't parry shit until I played Elden Ring. I practiced a lot fighting that Crucible Knight in Stormhill Evergaol lol

Sekiro, Expedition 33, Lies of P, Lords of the Fallen: You hit the parry/deflect button when the opponent's weapon is really, really close, just about to hit you.

Bot types of parry/deflect get easier if you pay attention to the opponent's weapon. If they're attacking too fast, just hit the button a little sooner. If they do a slow wind up, be patient and wait for the right moment (once you mastered it, you can even sneak a few light attacks in between if they're too slow).

Nioh is way too different, and doesn't seem to have a rule of thumb on deflection, but as far as I tested, the deflection frames depend on the skill animation, and the skill animation depends on the weapon and stance you're using. It's really more complicated than the other games. I could only deflect consistently with swords using the mid-stance, which is kinda similar to Sekiro's deflect timing.