I don't think the logic quite tracks there - an attack is, by nature, an instant effect. You call the power up and you let it fly. So it can't really have duration by default. Even the thaumaturgy "firebomb" trap wards don't really have an effect duration, per se - they just have an amount of time that they store the energy in the trap. When they go off, though, the attack is instant just like with an evocation.
However, if you wanted to allow it anyway, my only suggestion would be to stipulate that having the energy available to make an attack doesn't mean you automatically hit with it. So if you have a Weapon:4 evocation that you're holding for an extra exchange, you still have to make an attack roll with Discipline to see if you can put it on target.
But by default, yeah, no extending duration on attacks.