"...most athletes will see a benefit from an individual workout 10-14 days after it is completed." -Joe Rubio, a two-time U.S. Olympic trials marathon qualifier and head coach of the ASICS Aggies
Good article in Running Times How Long Does it Take to Benefit From a Hard Workout? that discusses some of the other factors that may be involved.
The experts agree that your last VO2 max workout (something like 5 x 1000 meters at 5K pace)should be eight to 10 days prior to an important race.