So, here's how I progressed with it: (all done in bare feet on grass, at end or in middle of a run)
Jun 18 - walk 5 minutes
Jul 3 - 2 x 2 min, with1 min walk (i.e. recovery between)
Jul 5 - 3 x 2 min, with 1 min walk
Jul 7 - 4 x 2 min, with 1 min walk
Jul 10 - 4 x 3 min, with 1 min walk
Jul 13 - 5 min, 3 min, 3 min, with 1 min walk
Jul 19 - 10 min
Jul 22 - 15 min
So it took me a month to get to 10 minutes of continuous running, but it is always better to be extremely patient with adding barefoot, even without a sore foot. With having tightness in the PF, I knew I could make things a lot worse if I did too much, so it wasn't hard to hold back and make sure it was helping, not hurting. I had three physio treatments for my PF as well, and am feeling quite good about how things are coming along. I'll probably do another 15 minute barefoot run next time, before hitting 20 minutes and then holding it at that level after that. For what I want out of it (injury prevention in the form of stronger feet, stretching out the achilles and calves, and a bit of help with form) I think that is enough time. Occasionally I might do 30 minutes, but that's about it.