I let mine swim for 15 - 30 minutes. I let the water get just deep enough they can lift their feet and swim but if they need to rest they can tip toe. Once they eat then have a swim and poop up a storm in the sink water (ewww LOL) they snuggle under the heat lamp and sleep for 2 hours. LOL
I let mine swim as early as 1 week. Nice warm water. When they have played and seem to be getting tired I towel them off and put them in a nice warm brooder. They love it.
You cannot let them free swim at will. They can drown. Just take precautions.