From Dark and Darker Wiki
For all damage-over-time and healing-over-time effects, added damage/healing is always added to the total. The game figures out the total damage, then divides that into ticks for base effect length, i.e. If you have 10 magic healing stat and cast Nature's Touch, the target will heal for 20 (15 base + (10 * 50% scaling)) divided by the base duration, 12 seconds, for 1.67 HP per second. Then you can use Buff Duration to make it last longer. If you had 10% buff duration, it wouldn't make you heal for the same amount over a longer time period, it would last another 1.2 seconds at the 1.67 HP per second rate, totaling to 22 HP over 13.2 seconds.