The SPF number only works if you apply enough product. A common practical guide is around 30 ml (about six teaspoons) for a full adult body wearing a swimsuit. For the face and neck, use a smaller but still generous amount, usually around half a teaspoon. If you use 30 ml per full-body application, a 120 ml tube gives roughly four full-body applications.
Do not expect one layer to last all day. Reapply about every two hours whenever you are outdoors. Reapply sooner after swimming, heavy sweating, towelling or rubbing the skin, because those actions break up the sunscreen film. The amount matters more than the number on the label: a thin layer of SPF 45 can actually deliver much less than the stated protection.
A useful habit is to apply before you go outside, on clean, dry skin. Spread the cream evenly over areas people often miss: ears, back of the neck, tops of the feet and the edges of clothing. If you use other skincare products, apply the sunscreen last, and give it a few minutes to settle before putting on other products.