I guess that depends on how you shoot. I've NEVER subscribed to the "how long can you hold" theory. What I use and teach is bone on bone structure to hold the bow open. IF you get bone on bone (skeletal alignment), you can hold all day, or until your muscles can no longer hold the bones in alignment.
Think about your legs when you are standing up and still. How long can you stand there? Why? You have your leg bones supporting your upper body weight, the muscles of your leg are just keeping the bones lined up. If however, you go to a half squat, then how long can you hold yourself up??
SOOOO many advantages to bone alignment with NO negatives, IMO.
Arne