I have seen the 3 minute quote for the +16 lbs, which is more than likely what was "official" as the max allowable without risk of engine damage. I have also seen sources that give it 5 minutes.
Now...I will also point out that in Aces High the "computer game", ALL of the engines have a simple "wep" system of: max wep power-overheat-wait for cool off-max wep power system. Either 10 minutes or 5 minutes. In other words, your LW fighters MW50 *NEVER* runs out, ever thought of that?
ALL of them, German , Japanese, British, Russian, American: MW50, Water Injection whatever. So dont go on some "oh I looked and found out that the Spitfires engines doesnt work EXACTLY like in real life so its porked" nonsense. You want 100 percent accurate engine models, fine by me, bring it on.