For me, it depends how quickly it happens ... if it's in the first minute or two, I'd change the water ... I like to let Darwin soak for 20-30 minutes.
This is new to me but in the last 2-3 baths he went to the bathroom about 5 minutes after being in the water... and I think I saw him soaking it up after. Why would that be?!?!
I think the warm water stimulates elimination, and at the same time absorbs into their skin and possibly reminds them that they're thirsty.
My Darwin is tiny, tends to poop tidily, and I soak him in a 12" underplate, so I normally just ladle out the poop and flush it while allowing him to continue his bath undisturbed.
Tortoises that get regulair soaks will get rid of the water they hold in their bodies. And then drink to get fresh available water.
In nature they can hold their water for a long periode of time. Cause there is not always fresh water available.
My Stars do it every time they soak, so every day.
I would soak your Tortoise for at least 30 minutes or more. And change the water when it gets dirty. You want your Tortoise to be healthy. So it takes some work. *glimlach*