After hatching, how long before they spin cocoons? I only want to keep silkies as feeders (and not breed), but I want to feed them all out before they spin.

Any particular signs that would indicate that stage is nearing - or would just counting the number of molts (I think I remember hearing 4 molts before they spin) be an indicator? I just want to plan accordingly so I can figure out how many to order and when I have to use them up by before they spin. Thanks!