Font of Life, does it activate once or multiple times?

I don't have buddies to test this with, my question is if the rune of "font of life" can be procc with as many auto attacks you are able to hit on one mark, or if it only works for one sole auto attack per mark.

example: I apply a mark/slow, would my adc only benefit from one auto attack consuming the debuff? or would he benefit from as many autos he lands before the 4 seconds expiration of the mark?