Personally I think any amount of time you are gone and leave a child unattended in a bath is potentially too long. Its not just drowning to consider, there are often products in a bath room that kids can get hold of, slipping and hitting their heads, turning hot taps on. etc. I burnt myself last week by accidently turning the hot on instead of the cold. The thing is you dont know what your child is capable of doing until they do it for the first time. So although you think your child cant turn on a tap, one day they will be able too.
DS has also slipped and hit his head on the edge of the bath, while I was in the room. Luckily I grabbed him and broke his fall but it could have been a lot worse.
I dont think I would leave my child unattended until they were at least 10. But personal choice.
I think people also feel "safer" leaving their children unsupervised in showers but it can happen. We had 2 children drown in the shower over here last year.