I normally only hold 1-2 seconds at anchor, but I like practicing holding my bow as long as possible,without shaking, a couple times a week. I do it for two reasos;
1) It keeps me from rushing my shots (I'm far less accurate when I rush).
2) I have had many situations where if I could have held just a little longer I would have had the deer dead to rights.
All that being said, I think the important idea here is not shaking, because there is no way to get a good shot when you do. Also, I agree with everyone above that you have to buid up to it just like weight lifting(drawing a bow uses different muscles). Even with practice I can't hold the bow more than 12-14 seconds before the shaking sets in and I only shoot about 43 lbs...JMO...Charlie