There's already rules on that. Generally the stronger it is, the more time it takes to charge. This is offset by how many powers you have as well, more powers = more charge time for each individual.

Your tier also comes into play.

Comparison: 
Erde, a character with one focused power. Most actions are immediate and as natural as moving your arm.

Ysolda, a character with a mile long list. Can cast them immediately but has a limiter on the number of times it can be cast before recharging.