When did God tell Satan that the seed of the woman would crush his power?
God revealed this to Satan immediately after man had fallen, announcing the coming day when the seed of the woman would deal with, defeat, and crush Satan's powers. That crushing was potentially accomplished at the cross, though Satan remains the god of this world and not yet under Christ's feet.
Les explains that as soon as God revealed to Satan that even though man had fallen, the day was coming when the seed of the woman would deal with the powers of Satan, defeat them, and crush them. He notes this was at the cross so far as the potential. But even after Christ was raised from the dead victoriously over a defeated Satan, Satan is still the God of this world, still destroying lives, and not yet under Christ's feet. Christ is to sit at the Father's right hand until His enemies are made His footstool, and Hebrews shows Christ has accomplished the work of the cross and risen from the dead.