How precise is the Deflect Mechanic in Sekiro?

Per the Sekiro wiki on deflection, the default time window to deflect an enemy's attack is 30 frames. At 60 FPS, that's a pretty generous half-second.

However, this frame window decreases if the player has recently just released their guard button. That decrease stacks, and the longer the player goes without a successful deflection, the smaller the window to deflect an attack becomes, down to a minimum of 7 frames. This mechanic exists to prevent players from simply spamming the guard button to get away with imprecise deflection.

Successfully deflecting an attack resets that frame window decrease back to a 30 frame deflection window. Thus, successfully deflecting multiple attacks in a row imposes no penalty on the deflection window (and is used multiple times in-game against high attack speed enemies). This reset rewards players who successfully manage to deflect an attack, even if they might have been guard-spamming. The deflection frame window decrease is also reset if the normal period of time for a normal deflection (30 frames) has passed without the player releasing the guard button, which, from my interpretation, is the game recognizing that you're simply no longer guard-spamming.

TL;DR:

The deflection window lasts for 30 frames (about one half-second). This is lower, down to 7 frames (less than one tenth of a second), if you're spamming the guard button and are not successfully deflecting.