A senior colleague from PoN tells me that a calcination of the type we need to do in the lessons takes maximum 2-3 hours.

1. Light the herb which has been drained.
2. Heat gently in a Pyrex Vision dish, stirring with a glass rod to ensure all alcohol is burned.
3. Heat more energetically at that point to calcine the remainder.
4. Continue until black residue is eliminated and the result is light coloured.

If in doubt, calcine longer rather than less long.