Shon, this answer is going to vary from person to person, but I'll tell you my experience.
I am very fair skinned. I had a friend who looked at my bare legs once and said, "You aren't white. You're mint green!"
It is impossible for me to tan, although I still have a smattering of freckles leftover from my childhood. So that should give you an idea of how pale I am.
For me, sunblock is not a seasonal thing. If I'm not wearing sunblock, it is possible for me to get a sunburn on my face in January in less than 30 minutes if I'm outside in a sunny spot. I have a habit of always wearing at least an SPF 15 on my face and that works fine for me for most of the year. When spring arrives and I'm wandering around in short sleeves, I'll put that same SPF 15 on my arms, too. I only think about applying something stronger than that if I'm doing any sort of outdoor activity where I know I'll be in the sun's direct rays for more than, say, an hour.