When do you start to let you child bath alone?

DS is great in the bath but he stands to get his toys which make me really nervous that he is going to slip and bump his head or something. I have been running out quickly lately to grab his towel if I forgot it or get DD if she is crying but I just cannot leave him to have a bath on his own. SIL has let her children bath on their own since they were a lot younger than DS (they would have been just over a year old). So am I being overprotective? What age was your child/ren when you started putting them in the bath and going to do something else? I haven't posted this for ppl to feel judged by anyone, just curious what other ppl do.