It's an exponential back-off kind of a thing. Our 6 month old probably requires 30 hours/week by himself (and that's already much reduced from what it was). Our 3 year old probably requires 10 hours/week. And our 4 year old maybe requires 3 hours per week.
The bigger issue is that this time is spread out pretty evenly throughout the day, so in practical terms it takes up every single hour that the kids are awake. What changes is that, as they get older, you have an increased ability to do other things in the in-between moments.
I easily put 30 hours a week in getting my 3 yr old dressed, to daycare, entertained while my wife makes dinner, and on weekends. My wife puts in similar time.
I guess we just live a different lifestyle. My wife is a full-time mom, so no daycare. The three year old dresses himself at this point, plus he and the four year old entertain each other pretty well.
I think there's also an element of once you have multiple children you just don't have any extra time. As a result, all the inessential time expenditures get cut. For instance, neither my wife nor I spend any time whatsoever entertaining our children. We don't have the time and they can entertain themselves.
The bigger issue is that this time is spread out pretty evenly throughout the day, so in practical terms it takes up every single hour that the kids are awake. What changes is that, as they get older, you have an increased ability to do other things in the in-between moments.