Wow! Great question!
Correct "timing" is when the hammer reaches full cock and the bolt locks the cylinder simultaneously (sounds like a single "click"). When that happens, you know the hand is the correct length. Next, the bolt " drop" (when the bolt drops off the hammer cam) should be near the beginning of the approach (lead) for the locking notch (if one is available, ) or at least one bolt width before the notch. A bolt dropping any later than that would be "late" bolt timing. For competition revolvers where speed is an ingredient, the bolt drop should be set slightly early - such as a bolt width and a half.
Also, during bolt "pickup" (the beginning of the cycle) the cam should pick up the bolt almost immediately to unlock the cylinder for chamber carry up by the hand. If pickup is late, you can feel a hesitation (bolt head not completely clear of the notch) or the cylinder may not even unlock if it is too late.
Listening for the two "clicks" as you lower the hammer from full cock, the first will be the hand selecting the next ratchet tooth and the second one will be the "reset" click. Reset is when the left bolt arm snaps back over the hammer cam (after being pushed to the side by the cam on the way down) so the cam can unlock the cylinder for the next cycle. That's where the "almost" comes from for the "pickup" comes in. Reset has to happen before the hammer face makes contact with a capped nipple,
otherwise, the hammer may be locked down on a live round.
The lock may be an old design but with an action stop and a bolt block installed, they are as accurate as any fine close tolerance instrument.
Mike