Probably more than you are at the moment! Most people do not use enough to achieve the quoted SPF. Some people suggest a shot glass worth, or a teaspoon per body part such as arms, legs, face and each side of the body, but I find these difficult concepts in practice, so I usually advise people to put enough on so that the skin looks completely white before the cream goes in.
It should be applied about 20 minutes before going out and then every two hours, or after swimming or towelling.
