You can sand as soon as the resin cures to a hard
state...There are many variables in the timing of resin
cure with both polyester and epoxy resins (temperature, humidity, type of
resin, etc) so it's better not to rush and just make sure the lamination is
hard.Note:
Polyester resin will still be a bit tacky after curing... you can still sand
it but make sure the resin is cured and
stiff.Epoxy resin will be nice and
hard and sand easily