Five seconds after being struck in combat, you will start healing at half your regen rate. After an additional ten seconds you will start healing at your full regen rate. Your full regen rate is 0.5 health per second, plus bonuses from powers, spells and equipment, with your green magic then being applied as a percentage bonus to the end result. Fractions are stored and added later.
For example, if you have 4 ranks in Regeneration and 100 in green magic, your full regen rate would be 0.5 + 2.0 = 2.5, modified by green magic = 5 health per second. Your half regen rate would be 2.5 health per second, meaning that you'd heal 2 health the first second, 3 health the next, then 2, then 3, etc.
Please note that your regen rate will be reset the moment you are successfully struck by another player, regardless of whether any damage was actually caused by the blow. If your regen rate is 1.0 or higher (0.5 base plus at least 0.5 from powers, spells or equipment) then being struck will only cause you to fall back to your half regen rate, unless the blow was a critical hit.
You may also eat food to recover health more quickly (see 'help eat').