SPF 50 is measured in a lab using a specific amount of product, so the protection you get in real life depends on how much you apply. A common rule of thumb is to apply about 2 milligrams of sunscreen per square centimeter of skin. For an adult body, that works out to roughly a shot glass (about 30 ml) of sunscreen; for the face and neck, about a teaspoon (about 5 ml). Since this tube is 150 ml, one full application uses about one-fifth of the tube.
Apply the cream generously to all exposed skin before going outside and give it a few minutes to settle. Reapply at least every two hours, and more often if you are sweating, swimming, or drying yourself with a towel. Even a high SPF product can be diluted or rubbed off, so a thin layer will not give you the full SPF 50 protection.
For a short everyday walk or a short commute, one application may be enough if you are not outside for long. For a full day outdoors, take the tube with you and reapply. The product also contains panthenol and vitamin E, which may help the skin stay softer after sun, but they do not replace the sunscreen.