I leave DD for a few seconds only to grab the nappy bucket or laundry basket. Our laundry is in the bathroom so I normally put the washing on while she is in the bath. If I do have to leave the room for a short time I get her to start singing a song so that I can hear her singing while I am out of the room. I have done this since she was about 2.5yrs. I don't think I will leave her alone for more than a minute until she is at least 5. She is pretty good and doesn't muck around in the bath but still I get worried.