When the sun should know very well what kind of skin you have to not encounter problems you would have to spend the holidays under the umbrella and towels several pitches for the sun do not touch your skin.
The sun, although it is good for the body, can also be detrimental to what type of skin as it is something you need to know before you can get in the sun.
So if you have a very white skin is normal as you have to use a high protection factor and prevent dry skin and the effects of the sun on it before thinking about tanning.
If you have a medium skin, i.e., it has color but not to say that is tan or white, then you lose some level of protection and you can take the sun for twice as long than if you had white skin (they can take a maximum of 15-20 minutes), so 40 minutes is more than enough.
Finally, if you have dark skin, without being tanned, the level of protection that we recommend is the same as a medium skin (and scroll down). The maximum exposure to the sun will be 60 minutes long, it is the skin, nourishing (water, food, etc…).