Looking to build a defensive DK character which makes relies heavily on health regen, but does anyone know how the numbers work? I'm guessing 25% health regen means 25% of your total heath but how often does it regen and what is the max?

No, if you get a bonus of +25% health regen, that means that you gain 25% more than your normal regen amount. If you were gaining 40 health/2seconds before, then you'd gain 50 instead. Be aware that base health regen is extremely low.