Longest I ever held at full draw was 20 seconds. A 10-pt was coming in just perfectly to my shooting lane. As he passed behind some brush before getting to the lane, I drew. Then, he stopped and just stood there, not spooked or anything just looking around. Finally he stepped forward and I put an arrow through his heart.

Other than that, I' ve never seen any advantage to holding at full draw for a longer period of time.