In many cases we see that devil fruit users go easy on their powers.
For example, why didn't Whitebeard spam his Gura Gura powers during the war? couldn't he just have destroyed the entire island? or simply from a distance cause massive sea waves on the island before even starting the war?
There was a scene when the three admirals had to block his attack, why didn't he try it again, and again?
Luffy doesn't seem to get tired of using his Gomu Gomu powers, he basically gets tired of punching, running, or fighting..etc
So what's the deal with how Devil Fruit users use their powers?