Your question contains it's own answer. Hours and Hours and Hours of ammo -- just depends on how you shoot.
One of my pet peeves is that there are a lot of folks that think they just blazed away. Untrained pilots did just this. A veteran pilot almost never ran out of ammo because he didn't use much. Guys like Ball and McCudden would often open up between 15 and 30 yards and fire just a few rounds.
But I will address your question just for the sake of argument where two guns were concerned:
Lewis gun Mk II* in British Service (nope were not all the same): 97 round magazine generally used with this weapon. Not used by the RNAS until after April 1, 1918. Rate of fire was 700 rounds per minute. Which works out to about a little more than 8 seconds of firing before you have to change drums.
It appears the RNAS may have been using, primarily, the MK 1 gun which had a rate of fire of 550 rounds per minute. Which would give it about 10.5 seconds of fire before you had to change drums.
There are many accounts of a gunner stating he "emptied a drum" when he fired. I take this to mean (at least some of the time) that the gunner fired the entire drum in one long burst.
Vickers Mk 1 & MK II -- British Service
850 rounds per minute with the Hazelton booster.
The problem with this was the interrupter gear -- which was the rate limiter. So the early interrupter gears would limit the rate of fire to something less than 450 rounds per minute (Sopwith Kauper Gear and Ross Gear).
Just running through my notes I'm not finding how many rounds per gun. The Germans carried 500 rounds per gun, so I will assume this is the same (probably varied by the plane and country). So you would get roughly 67 seconds of fire from an early Vickers. The Clerget Camel used the Sopwith Kauper Gear till the end of the war.
Later interrupter gears would push the rate of fire into the 750 rounds per minute range (some sources say more but . . . personally I think this is just British propaganda).
So if we use that figure, you would have about 40 seconds of fire.
Finally there is no way no how you could sustain that rate of fire without damaging the guns or jamming the guns. So it is unlikely that 'maxim' rates of fire were ever attained in a belt fed machine gun.
mjc
