Studies show that the correct amount of sunscreen applied to the body for it to actually work as directed is 1 ounce. If you need a visual, that's about a shot glass of sunscreen. You're also meant to reapply it every 2 hours, regardless of whether or not you were sweating or swimming. That means if you have a 6 ounce bottle, it should only last you about 12 hours in the sun, not all season!
If you're relying on make-up as a sunscreen, you need to apply an entire teaspoon of liquid foundation for your face to gain the full benefit (who the hell wears that much foundation?!) and if you use powder you need to apply a gram of it or half a compact worth. wtf.
Now of course if you're only getting minimum exposure, and only some of you is even being directly hit by the sun, you can get away with less. If you're at the pool or the beach however, you better stock up! And remember, sun can damage your skin through clouds and glass so apply appropriately even if it's overcast or you're just spending a day at the office in front of your big picture window.