Not sure when we started doing it, but we leave ours and they're about 3 and 18months.
I'd say it depends on each situation - in our situation, our house is quite small, and the kids quite chatty, so when I hear them stop chatting, I'll stick my head in. And the bathroom's quite central so I'm walking past quite often.

I imagine it'll be different in our new house where the bathroom is tucked away a bit more, I might have to find something to do much closer. And obviously I don't go out of earshot, like outside or anything. I don't think I'd be putting washing on the line during a bath til they were much older, as in old enough for someone to come and get me if things went wrong.