Yeh spot on Matthieu, probably should have been more clear with my response. I meant fully hardened rather than fully cured. As Shaneer said, 2-3 hours at 25 degrees is fine. Give it an extra 2 hours for every 5 degree drop in temperature, assuming you've catalysed accordingly.
Be a bit wary of bonding to the skin coat TOO early though, it's better to give it more time to cure than less time. The skin coat is a vinyl ester and the tooling resin is a polyester, and whilst they are both very low shrink they do still shrink and they do shrink at different rates. Bonding to the skin coat prematurely can increase the chances of distorting your mould as your resins shrink. Remember you don't really have a window of time for bonding as you do with the gelcoat, you can leave the skin coat for a year and all it takes is a wipe with acetone and a good scuff up with 60 grit before you can bond to it again.