I've had this, and it can be really painful. I can't give a clear guideline on how long to rest, but it hurts, you need to stop. If you try to push through it, it's going to continue to hurt.
As far as stretching/strengthening goes, the condition seems to be caused by an abnormal change in running form. You may want to compare the strength in each leg and work on building up the weaker leg.
I got it because I had a broken kneecap that required surgery and 3 months in a cast. I lost 6 inches off the circumference of my left thigh. I tried coming back pretty quick and ended up with the condition. Once I got the left leg strength back, I haven't had much trouble with it.