Light Aircraft: Gallons (usually US Gallons) per hour
Medium Aircraft: Lbs/Kgs per hour
Heavy Aircraft: Tonnes per hour!
Note that these are all time-based. Miles (nautical usually) are of little use as you will cover more of them for each unit of time with the wind behind you than you will with the wind against you, but the engine(s) use the same amount per unit of time regardless.