How long does a walk in bath take to empty?

We have tested the baths ourselves. From being completely full (i.e. nobody in the bath, so maximum water capacity) a 200 litre twin waste bath takes about two and a half minutes to empty. Bearing in mind that the person would displace a considerable amount of water, you can expect emptying to be faster than that. Of course, drainage will depend on your property and the system, but this is a useful guide.