The April 2012 issue of Running Times buried the lede, but in the "How to Improve" section of the "Dorsiflexion of Ankle" portion of the "Are you Ready to Go Minimal?" article, they say:
"Hold [the stretch] for three minutes on each side. (Yes, three minutes is how long research shows we need to hold stretches for tissues that are too short.)"Now, I'm big into stretching, but that seems like a long time to have to hold a stretch, esp. for folks like myself who are pressed for time before and after running. However, if that's indeed what it takes to remold your muscles, i'll have to start devoting the extra time to stretching the muscles I want to lengthen/loosen post-run - mainly my calves.