How precise is the Deflect Mechanic in Sekiro?

The "Deflect" mechanic in Sekiro can be pretty precise - if you spam the button, it doesn't always work, but if you learn the timings as intended it can be 100% effective.

Just how precise is it, in terms of frames or seconds?

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.