Applying sunscreen every morning should be an essential part of your daily routine. In fact, anytime the UV rating is above 3 (which it is in post parts of Australia every day of the year, you should be wearing an SPF.
Up to 90% of skin aging and most skin cancers are caused by unprotected sun exposure. The damage is cumulative, and in Australia, can occur in as little as 10 minutes in the sun.