Since your questions seems to be motivated by personal health protection, consider an alternate view, which is that not only is the sun's damage to the skin not "sufficient to warrant protection anytime I go outside or just for extended periods", but that lack of sufficient unblocked exposure to the sun may be, overall, even more harmful--not only for the risk of various cancers, but for a large number of serious diseases.
Dr. Michael Holick is a Professor of Medicine, Physiology and Biophysics at Boston University School of Medicine, and has researched vitamin D--which is produced by our skin when exposed to the sun's UVB radiation--and its critical role in health for over 30 years. In addition to his book, The Vitamin D Solution and an entertaining and informative (and at times, zany) lecture, he recently (Jul 24, 2015) wrote about this in The Washington Post. For example:
the risks associated with sensible sun exposure have been exaggerated
by well-meaning health authorities, and the measures to guard against
them often have nothing to do with the sun’s occasionally malignant
effects. Contrary to the paranoia generated by years of messaging, the
sun is not our enemy. It’s safe to step back outside — and, please, go
easy on the sunscreen.
The book and lecture provide much more detail, but the main points are that 1) vitamin D, a hormone, is critical to many bodily functions, 2) that lack of it in a majority of people in many countries is a "pandemic" related to many important diseases, 3) that the current United States 400 IU/day guidelines are too low, 4) that it is easily produced by sufficient exposure to unblocked mid-day summer sun, and, importantly, 5) that there may be additional "photoproducts" (other molecules our skin manufactures in response to UVB rays) that we may find are similarly important to cellular processes and so he prefers sun exposure to taking supplements. In regards to diseases that are related to vitamin D deficiency, he writes:
A lack of vitamin D is associated with increased risk for Type 1 and 2
diabetes, multiple sclerosis, rheumatoid arthritis, Crohn’s disease,
cardiovascular disease, stroke, depression, Alzheimer’s disease,
schizophrenia, colon and breast cancer, influenza and tuberculosis....Of the 30 leading causes of deaths in the
United States in 2010, 19 were linked to low vitamin D status.
He does not deny that one needs to limit exposure to the sun reasonably, and recommends sun block for the face and basically exposing arms and legs something like 10-20 minutes at a time, several times a week, from 10am--3pm (when UV-B is sufficiently strong, else one only gets UV-A, which does nothing for vitamin D but is linked to skin damage), though exact "dosing" will be dependent on latitude, skin color, time of year, and perhaps other factors (which he covers in the book).
