I swear I read it had 9 min but I rechecked my sources, I stand corrected..
Considering the D40 i is 20ENY and the N is 5 with no appreciable improvement in 90% of circumstances, I seldom fly the N
The P-47N actually had a water tank that would hold 35 gallons of water. At the consumption rate of the water injection system, this meant 15 minutes of available water at 72" of MP. And, at MP's between 54" and 72", the water consumption increased or decreased proportionally. So, again, theoretically, at 60" of MP, you would have more than 15 minutes of water available. Of course, the engine was still limited to the 5 minute of continuous WEP, meaning you couldn't, theoretically, run all 15 minutes of water through the engine in one shot, as you would risk damage to the engine.
The important thing to note is that the AH FM for the American water-injected planes works independent of what the real-life capability was, so we still only get 5 minutes of continuous WEP, even though theoretically we should have 15 minutes available.