Apply sunscreen 20 minutes before going outside at least.

Sunscreen needs to be applied 10 to 15 minutes before stepping out into the sun and any sunscreen usually have an effect for 3 to 3 and 1/2 hours. Also the same sunscreen might not suit everybody and the right sunscreen to be used depends on the skin type. The skin has to be examined and assessed to know which one would suit you best. Also, you can read my blog to know the importance of using sunscreen routinely. https://www.practo.com/healthfeed/need-for-protecting-your-skin-from-the-scorching-sun-28406/post. Hope this is helpful.