You should run your lawn mower out of gas for winter storage.
The tank should be dry and the carburetor should also be free of gasoline to prevent the carburetor from gumming up.
Before storing my push mower and riding mower I run them until they are completely out of gas which keeps the carburetor clear of any gum buildup which can mean a rebuild of the carburetor if it does gum up.
Old gas eventually varnishes and clogs carburetors so it's best to always run the law mower out of gas before storage.
The amount of fuel that a lawn mower uses per hour depends on the lawn mower and the size of the engine.
Most riding lawn mowers use around 1.65 gallons of gasoline per hour.
Most diesel lawn mowers use around 1 gallon of diesel per hour.
The lower the engine RPM the less fuel it will use and the higher the RPM the more fuel the lawn mower will use.
A push lawn mower often uses a 1/2 gallon of fuel for an hour of run time.
Riding mowers have larger engines and the higher you have the throttle the more gasoline it will consume.
Also if you're cutting tall grass the lawn mower engine will often use some more fuel to power through the grass.
Some larger zero turn mowers can use up to 2 gallons of gasoline per hour.
An average riding mower holds around 1 to 2 gallons of gas although larger riding mowers may hold up to 5 to 6 gallons of gas.