SPF or sun protection factor is a measure of protection against only UVB.
An SPF of 30 means that you will be able to stay in the sun 30 times longer without burning, than you would be able to without wearing the sunscreen.
This does not equate to 30 min, and most importantly is different for each person, as we all burn at different rates depending on our skin type.
SPF is measured in a test environment which is nothing like real life.
It does not take into account sweating, environmental factors and the varying application by the sunscreen user. In a test environment sunscreen is applied very thickly, at 2mg of sunscreen per square centimeter. The quantity that is actually applied by people is closer to 0.5mg per square centimeter. So we are never really getting the same protection as on the label.
While it is true that SPF15 filters out 93.3% of UVB, SPF30 96.7% and SPF50 98.3%, making it seem that there is very little benefit in using the higher SPF, one needs to consider that only the amount of UV light reaching your skin is important. If you look at it this way then SPF 15 allows 6.7% of UVB to reach your skin while SPF 50 only allows 1.7% of UVB.
Dermatologist associations worldwide recommend you wear at least an SPF of 30.
SPF says nothing about the UVA cover of a sunscreen. There is at present no universally applied measure of UVA protection. Look for UVA with a circle around it on your sunscreen bottle. This denotes high UVA protection.
A final word is that we must remember that 80% of sun exposure is incidental and happens during the course of our day to day life, while we are not actively “lying down to tan”. UVA penetrates through windows and causes sun damage without causing sunburn.
If you find that you are getting sunburned, and notice the increasing appearance of “sun spots”, you are getting more exposure than you thought and need to re-look at your sun protection measures.