I took four days off from running after my marathon. I didn't have a set time frame in mind, I just played it by how I felt. I was sore the first three days, felt better on the fourth, and decided to go on an easy trail run on the fifth day after my PR marathon. My left hamstring was extremely tight that morning and I hoped a run would loosen it up. 2.7 easy miles on the trail made me feel much better. The next day I tried 5 easy miles on the trail; my left IT band started hurting a little at 3.5 miles so I took the next day as a rest day.
Today I ran another easy 5 miles on a different trail. No walking and no pain.. except.. Yes, 3rd time is a charm.. I tripped and got back up immediately without looking at my knees until afterwards. Oh well.. a scraped knee is totally worth a heavenly trail run :)