If it is a hamstring it depends on the damage, you would have to get some feedback off a specialist.
I had a ham tear in May - I rested for a week and then did some very easy runs and built up gradually. I did race in early June (because I really wanted to do Blencathra) but took it very steady and got round without further damage and then it just went back to normal.

Stretch, stretch, s t r e t c h.