My commute is 35 minutes each way and being at opposite ends of the day there is enough recovery time that I don't ever burn or tan. I'm sure a dermatologist would tell me that I should cover up because that's what they do, but I just ride in shorts and tech-t and my helmet.
If I'm going to ride any longer I will apply sunscreen everywhere that is exposed including top of my head that is exposed by helmet vents. I've never burned in an area covered by my non-spf related technical T's or jerseys, no matter how long the ride. I'm not really sure if SPF clothing is really beneficial or if it's just a marketing thing?