OP here, home from a long day at work and ready for my baby to wake up and eat some dinner so we can make a big mess and then play in the bath!
See, I really meant it when I said that I enjoy dd's bathtime, and that my question was mostly for curiosity and future planning's sake. And I really appreciate all of the thoughtful and useful answers that have leaned in that direction...
What stands out to me the most about these responses is that I see good moms who know their children and who give thought-out answers that make sense, yet those answers run the gamut from 18 months to six years, and even extend a little bit beyond those boundaries.
I know my daughter will be able to take a bath by herself before she is four or five... but it is reassuring to know that it is not abnormal for me to sit right next to her while she plays in the tub now. And that there are at least some moms out there who won't think I am an awful person if I leave her side for ten or fifteen seconds, whatever my reason might be.