I remember an explanation on the FAQ of Wizards I think, you turn until you have no more turning left at the time you want to turn.
18 charisma=4+3 base=7.
If you increase to 22 Cha, you gain 2 more TU, but if you don't burn them before the end of the spell you loose them, if you make 8 TU during the spell when it's gone you have no more left for the day, unless something can grant you again more than 8 TU.
Maybe that's nor very clear.