How much gas am I supposed to burn for a 1999 Toyota Camry

It depends on many factors including: is it a 4 or 6 cylinder, are there any performance mods, when was the last tune-up/ oil change, etc. With that being said, check out www.fueleconomy.gov. If your getting poor gas mileage and your vehicle is stock, make sure to change the air filter, fuel filter, oil/filter, spark plugs/wires, check your tires pressure and tread wear, and lastly run fuel injector cleaner/ fuel system cleaner in your car. Just doing a few of these could help give 2-4+ mpg more. Good luck