When my boys DS1(3.5) & DS2(almost 15months) bath i am generally there for the whole thing. The only thing i may leave them to do is run around and get pj's organised in front of the hearter if i didn't get to do it before they hopped in. But it's only for the quickest time and i race back to them as soon as i can. DS2 is also a bit cheeky for standing up in the water which freaks me out.

DS1 i leave for minutes while i get DS2 out and dressed but i constantly call out to him and get him to talk to me. If DH is home they aren't really left alone to bath because we can 'tag team' but i think he's more nervous about leaving them for a second that i am.