Reading around the topic, here's something from the two time olympian:
"Before you start running barefoot, you should walk barefoot for a few weeks to strengthen the muscles in your feet and ankles and toughen up the skin on the bottom of your feet. Start out by walking barefoot for 5 to 10 minutes a couple of times each day. You can also do exercises while walking barefoot such as high knees and walking on the balls of your feet to prepare your feet for running barefoot. Dr. Stewart says, "Feel your arches the next morning. If they are not sore, then you can do a little more the next day, but progress slowly. When you can walk barefoot for an hour relatively comfortably, then you should be ready to start a little barefoot running."
Once you start barefoot running, the forces on your feet and calf muscles increase greatly. Start out with five minutes at the very most, and increase slowly, running barefoot every two to three days. After a couple of months, you may get to where you can run barefoot for 20 minutes, but for some people a few minutes will be all they can handle. "
This is more conservative than I've thought! But as Jon Tilt and I were saying a couple of weeks back, t&f athletes are probably in a good place at this time of year with regards their plantar facia and achilles, having been running in spikes from April through August (800m in my case).