I was gonna say the exact same thing...
Another option I'm toying with would be using Continuous Charges, starting with a base of 16 for unlimited. But since I want all spells to cost END, and restrict the duration some, all charges would cost END, with an additional END cost for every step down the time chart. EG: 20 AP of "light" for an hour costs (2*5)10 END. Increased Duration would lower the multiple by one.