Day-tanks located in the same room or mounted directly on a generator are typically limited by fire code to gallons (liters) of diesel fuel.

I believe the answer is: 60 (240)

According to fire codes 1910.106 (d)(3)(i) , "Not more than 60 gallons of Category 1, 2 or 3 flammable liquids, nor more than 120 gallons of Category 4 flammable liquids may be stored in a storage cabinet."
These flammable liquids would include things such as diesel, motor oil, cooking oil, mercury, etc.