"Within four weeks [the body] has probably made all the adaptations it's going to do. I never understand why people train every day the same way and expect to improve. Within a few weeks, or certainly a month or two, you're going to hit a plateau, and the only way you can break out is to change your training in some way." (runningtimes.com)
MY COMMENT: Food for thought. I am probably guilty of spending too much time doing the same workouts.