Amount of sunscreen to wear daily depends on where you live, what clothes you're wearing, and how long you'll be outside. Your Fitzpatrick Skin Type - how fair or dark your skin is - should also be a clue as to how much protection you need. If you're in Miami in the middle of summer and you're outdoors a lot, you'll need a stronger sunblock, applied over more of your exposed body, than someone wearing a suit in New York City on a cloudy fall day. If it's bright and sunny and you're outside for more than a few minutes, wear a sunblock on exposed areas of skin (including hands, ears, neck, chest, arms, and legs). Unless you're in direct sun all day long, you probably don't need to worry about reapplying sunblock on your body, but do put it on before you go out. Don't overthink it or rationalize that you might not need it. Remember, the sun's rays are killer laser beams. If they're going to be hitting you, protect your skin with the sunblock.
It's mandatory to apply a sunscreen every single day - no matter what - on your face, neck, ears, and upper chest (if it's visible). If most of your body is covered with clothing (which usually has an SPF of less than 10) and you spend much of your time inside, it's not necessary to apply sunscreen head to toe every day - although if you like a particular body lotion with sunscreen, by all means wear it every day. And use a hand cream with added sunscreen while you're at it, to help prevent the wrinkling and sun spots that are telltale signs of your age. (Most women's hands, chests, and shoulders look years older than their faces, just because they neglect to protect and moisturize them).