The 6/6 bonus for the Centaur set gives 10 seconds of swiftness, with a cooldown of 10 seconds; however the cooldown seems to be more around 15-16 seconds.
When using a healing skill I increase my boon duration to the point where I gain swiftness for 13 seconds, and when I immediately use my heal skill after it I have to wait another 3-4 seconds.
Thought it was worth bringing up 
