Metabolism lets you make a teammate's health regenerate at a faster rate than normal. The effect remains until they die. You cannot administer this buff to yourself.
This allows a Medic to increase another player's health regeneration rate, letting them return to full health 2 seconds faster. It is administered like a Life Buff, by injecting them with a syringe. As such, the Medic needs to be close enough to the target in order to interact with them. This ability speeds up health regeneration when the bar is filling. It does not speed up the waiting time for health to regenerate.