Remember that standing practice at it's core is isometrics. Properly done you should notice your strength and power increase over time. IMO, focus on this and don't get all wrapped up in the Qi cultivation thing.

I agree with Bob & Buddy in that 40 min. is max. When I trained in Yi Quan we would hold 8 postures a max of 5 min each. My teacher said that any longer was counter productive. One of the best write ups on standing practice I have seen is in the book Xing Yi Nei Gong - Xing Yi Health Maintenance and Internal Strength Development edited & compiled by Dan Miller & Tim Cartmell. Tim does a chapter on standing practice which is really straight forward & easy to understand.