And then I think, What?
Okay, it would be fine for a fireball, but there are few spells where a brief effect is all you want. Blasting something, mending something. relocating something (somewhere stable!) -- but little else. I don't know if it would even make sense to have it change colors or the like. The only lasting effects would probably be those with ontological inertia -- a fireball could light things on fire, which could burn on even if the spell ended, like a fire after the match goes out.
And I've already had one character teach the others a spell of certain duration that they cast with nothing but -- their wands.
And then I go -- Duh! That's why they use wands, and preferably of durable materials. True, to work more powerful or more durable or more delicate spells, you would need more elaborate prepartions, but to cast a spell defending you from a certain charm spell, or color your gown, or making something glow, the wand's durability will transfer well enough.