To download D Minder Pro by Ontometrics, go to the app store and type in D Minder -- or go to D Minder Pro on iTunes to read more about this great app that "will help you track and manage your vitamin D." There is a "Start Sun Exposure Button" where you can set the % of exposed skin, the weather and enter any supplemental vitamin D that you might be taking--then swipe left and press the sun button to see how much vitamin D you are getting when you are in the sun. You can see below the recommended time in the sun.
Of course, if you have a family history of cancer, mindfulness around the sun is even more critical. With a family history of skin cancer yearly or bi yearly skin checks with a dermatologist and follow their recommendations.
Even if you don't use the app to manage your own vitamin D levels you can use it to see the UVI index--which is also very helpful in term of knowing the safest time to enjoy time outside.