One hesitates to give such a wishy-washy answer, but it really does depend on how bad the damage is. I started Detroit one year and popped something in my hamstring just past halfway. I walked off the course, iced the hamstring, got physio treatment and was able to maintain fitness well enough with alternative cardio exercises that I was able to run my next marathon less than four months later. On another occasion, I was out for a couple of months, tried to come back too quickly and re-injured the hamstring and was another two months without running at all.
Speak to your doctor or physiotherapist and don't try to come back too quickly. For most of us, there are many more years of running ahead of us provided we're not silly enough to damage ourselves too much.