Leo actually looks a little bit thoughtful at that. He tries to imagine himself holding up his flame powers for that long—but that's not really comparable, isn't it? Frank's power is, admittedly, pretty cool, and Leo's is what a lot of people would call dangerous. Destructive.

So he shrugs. "Not bad."

And just because he can't help it: "You can probably do better." A smirk, but maybe he's trying to be supportive too, just a little?