When your character successfully defends against an attack, it takes some time before that defence can be used again - this is called 'cooldown'. For your feet (or unarmed attacks) this cooldown is between 4 and 7 seconds, depending on your encumbrance (4 for unencumbered, 5 for light, 6 for medium and 7 for heavy). For a weapon it varies depending on the weapon type, but encumbrance is also a factor - you can find out the cooldown time for a weapon by looking at it. Note that the weapon in your off-hand will also have +1 cooldown.
The cooldown timer won't go down while performing a combat technique (although it can still parry or block while being used to perform a technique - only the recovery time is delayed).
While wielding a weapon two handed, the assisting hand will automatically be used to help the cooldown timer for the other hand. Thus the cooldown for a two-handed weapon will be half the normal time, or normal time while performing techniques.
See also: defence