It's quite unpredictable, one day you can have no dark hairs and then a day or so later you might notice a bunch have appeared. The only indicator I noticed was that the blond hairs get thicker in the pattern where the terminals appear, and may or may not change color whenever they feel like it. It's quite slow for me at the moment, but all the ones that have turned into proper beard hairs did this before turning. They became hard and tougher first, the first part to turn was the upper lip area as well. Followed by sideburn area/chin/neck (in that order)