North American P-51 Mustang: approx. 30 sec of firing time
Guns: 6 × 0.50 caliber (12.7mm) AN/M2 Browning machine guns with 1,840 total rounds (380 rounds for each on the inboard pair, and 270 rounds for each of the outer two pair)
Rate of fire: 750–850 rounds/min (AN/M2)
Hawker Hurricane/Supermarine Spitfire: approx. 18 sec/19 sec of firing time
8 × .303 in Browning Mk II* machine guns (Spitfire - 350 rounds per gun) (Hurricane - inner guns 338 rounds, next gun 324 rounds, outer guns 338 rounds)
Rate of fire: initially 1,100 rounds/min
Given the above, 1 minute of firing time sounds pretty good.