I agree... however it also depends on the kind of resin that you are using. Above is correct if using Envirotex (mix 1/2 & 1/2 kind). If it doesn't harden after 2 days, it's not going to. Possible mess-ups are not measuring exactly OR not stirring enough (I stir until I think it's done, then stir some more).
HOWEVER, if you are using the kind of casting resin that you mix a large part to a few drops of catalyst, I have experienced the need to dry for a looooong time. I thought I had a ruined batch because of the ability of leaving marks, but 3 weeks later, they were perfect!
AND, regarding both kinds... make sure you are pouring in the right temperature zone. I've poured in too cool of an environment with bad results.
Wear your respirator!! -flip