Tough question. The safest answer would be once the area is no longer tender to finger pressure. Fortunately, if you are in the Northern Hemisphere, it is a bit easier to give a little more time to rest (than if you were, say, two months out from an event). A few further thoughts:
1) while laying off of running, you might try deep end pool running with a floatation vest - insanely boring, but you do replicate the neuromuscular patterning of running, and that may be of some benefit in forestalling some declines in your running
2) when you ultimately resume running, taping or orthotics are a must as you resume. Also continue the stretching. Icing may be of benefit after your run as well.
3) when you ultimately resume running, start with ridiculously simple goals for duration - test dose of running, check your response the next day, then proceed based on whether the plantar fascia is holding up or regressing
4) to facilitate #2, you might want to start your running on a treadmill - so you have more control over surface/canting/speed, and most importantly, duration. Hop off if you get any twinges.
5) When returning to the road or trail, be careful to avoid having your affected foot on the high side of a cant (it would be better to have it on the low side of the cant).
Steve Palladino
http://www.permanente.net/...tor/steve_palladino/
http://www.eteamz.com/...rt=24&id=4591042