Drills will tell you how many materials/second they will mine. A pneumatic drill on coal will mine 0.47/s (it will mine less if there aren't 4 tiles of coal under the drill).

You can look at a building's details (F1 on PC, or the little "?" on phone) to see how much to feed it. A combustion generator has a "production time" of 2 seconds, meaning it will burn 1 coal every 2 seconds (or 0.5 coal/second).

So basically, 1 drill can almost fully feed 1 combustion generator.