I know some people wear it all year round, but when do normal people with average skin start wearing it when going out on the beach? March, April, .. ?
People probably get most burned in the spring when it's still cool and doesn't seem "hot" enough to get a sunburn. It's always good to wear sunscreen - any time of year - if you'll be out in the sun for any amount of time - on the beach or wherever. I don't believe in massive sunscreen though - just enough to keep skin from a sunburn. Those with very fair skin need to cover up!
For me it varies a bit. By the time I'm ready to go out for extended periods it is time to lather up. Usually March. The sun is definitely intense enough to burn a normal person in March after about a half hour or so.
True story: a girl named Stephanie who lived in Atlanta, in preparation for her trip down here, spent two weeks of spray tanning sessions before laying out in the real sun here. First day down, she laid out most of the day and wondered why she had such a bad sunburn....she's a friend of a friend..