The longest my boat has ever been over was 10-15 minutes, which seemed like a long time to me but is apparently not that long compared to some of these stories. That said, one of my two dry bags was completely dry, the other had a smidge of water in it, but it had some oddly shaped contents so was hard to seal properly. The other two dry bags on my boat got soaked. I'd assumed it was because they were not properly sealed.
My dry bags are the ones available at Jax. Seal Line I think the brand is? Now I'm thinking I might do a test and leave a couple dry bags submerged overnight and see how they do. I've got a pickle barrel I could fill with water...