After building a new bike with different geometry, I'm able to stand longer. I have not measured but I'd guess it increased 50%.

On the new bike, the seat tube is more relaxed; the seat is further back related to the pedals. I've replicated the reach with a shorter stem. So when standing, where seat location is irrelevant, the bars are closer to my body. I think my upper body is carrying more weight out of the saddle since the bars are closer, and I think that had made a difference.