My yellow belly once sat in his dish for 13 hours, I believe it to be longer except I had to go to sleep. I woke up and he was still in it. He was in Shed though. When BPs get ready to shed they will start soaking. Once they shed, they just seem to drink the water vice sit in it. A long soak, in my opinion, is about 12-24 hours. Soaking too long can cause Belly Rot, and you dont want that. You have a dish thats just big enough for them to soak in, and only 1/2-1" of water, depending on the size of your snake.

Just my 2 cents.