I usually see mine not too long after they are born. Then, they seem to go into hiding for about a week or so, and as they get a bit older and braver, you'll see them out cruising around on the substrate. One trick that I do is wait for an hour or so after the lights have turned off for the night, and then shine a flashlight along the substrate or the glass side of the tank. If they are still very young and nervous about being out in the light, sometimes you'll see them being more active when the lights are out. After about 10-14 days, you should see them pretty regularly, and running out and about with the adults. There is no hard and fast rule, but this is generally what I see in my tanks. The more I just leave things alone and watch, the better off I am.