We all know that after refuelling the fuel has to settle before we're able to draw any water if there is any. But the reality is that this could take hours... days even.
I'm not sure it does, you know. If you introduce a drop of water into a jar of AvGas, it settles to the bottom pretty quickly. OK, in a large tank the fuel will tend to swirl around for a while, but I think it dissipates quickly too.
If there's a significant amount of water in the fuel, enough to affect your engine, say, 20 minutes after refueling, you should be able to detect some of it after 10 minutes, I think.