I am paranoid. I think 5 or so. I give both kids (3.5 and 15 months) a bath together. I take the baby out to get him dressed and make DS1 sing the whole time so I know he is OK. I then leave the baby in his crib and go rinse DS1 and help him out of the tub. I am too scared to let him get in and out by himself, even though I do leave him for a couple of minutes, I am just a room away.
DD is 3.5, and it will be a long time before I leave her, even to grab a towel. If I forget something, I take her out and make her follow me. I don't even leave her sitting on the toilet while the tub is filling. She's a rule follower, and so I know she's not likely to hop in while I'm not in the room, but she's also 3, so not an age for impulse control. I also don't let her out of my sight while she's eating for fear that she'll choke. Drowning and choking = silent, fast killers.
My son will be 3 in July and I am still way too paranoid to leave him alone in the tub for more than 30 seconds. If he asks for a specific car that is in the adjacent room, I'll quickly run and get it, but I don't generally leave for anything.
The laundry room is the next room down the hall from their bathroom, and they bathe together (nearly 2 and 4.5). I fold laundry and listen to them play. If I hear a thud or one of them gets quiet, I pop my head in. It helps that I can ask ds if dd is ok and he will reliably answer.