"I think I will be wrist-deep in doo-doo within 18 months. I'm calling it right here," Downey, 45, tells Playboy in its November issue.
Though it's not exactly clear when the interview took place and the countdown officially began, Downey and his wife Susan still have time to prepare.
Already father to 17-year-old son Indio – with former wife Deborah Falconer, whom the actor married in 1992; their divorce became final in 2004 – Downey is also considering the sex of his next born.
"I think we should probably try to have a girl, because I don't want another male entity to have to compete; I don't want Indio to feel there's another boy in my life. But I don't know what we'd have to do [to have a girl]. Do we have to put it in a spoon and hang it upside-down?"
Pressed for what he finds most exciting about being a dad, Downey declares, "The upside is doing what you're supposed to do, what feels righteous. And there is another 18 years, legally, of thankless blood, sweat and tears."