Last year we lost a good friend to melanoma, three months before their ONLY child was born. Sunscreen is non-negotiable in our family.
The guidelines are 1oz per application for an adult. That assumes full-body application, like if you were going swimming. If you're doing just exposed skin like face, arms, legs, neck you'll use about half that for an adult. Munchkins use much less!
The five of us (two adults, two teens and a 9yo) use about a can of spray per day when at WDW. If we are using lotion instead, we use about 5-6oz per day, since we apply sunscreen in the morning and before hitting the pool in the afternoon (not all of us swim each day). We use face-only sunscreen on faces (duh), though I use makeup with SPF.