When you buy it, it comes with a multilanguage leaflet that shows the time factors. 4 rolls requires no factor, after that a multiplier is used. I found that the most I needed at the time, was about 1.5 times. The factors are different for volume and tank size, so read the leaflet, and you'll be good to go. I haven't used it in a while, but when my HC-110 runs out, I'll be picking some up!
I may have been wrong about the 18 rolls, though, as I recall something about up to 60 rolls per liter of concentrate.