How will I know when my dreads are tight / mature? - Dreadlocks FAQ's
You will know.... =] They get hard like ropes because you can bend them and they are flexible but if you try to squish them flat they resist. They become much harder to penetrate with a pencil tip or even a paper clip. The amount that they can be squeezed down flat with about 2.5 lbs of pressure becomes very small, usually about 1-4 mm. Larger dreads get tighter than thinner dreads. Very thin dreads with sections smaller than 1/2" may not have enough hair to feel fully mature even when they have reached full tightness.