Huh. My answer (as a father of 3) to how much time they'll consume would, only slightly facetiously, be "all of it."
Your math about sleep cycles is way wrong, and the idea that you'll still have half the free time you used to is laughable. Assuming a more or less 'normal' work schedule, most of the time they're sleeping, this guy will either be at work or will be sleeping himself.
My advice is to assume that for a few years you're going to have very little downtime unless you don't sleep much or don't feel the need to see your wife much.
I'm a middle child, and yes I suffer from middle-child syndrome big-time, still. Working class background, mum was a housewife and dad worked in a textile mill. Its amazing how he managed to raise us on that salary, yet 7 out of 10 of us have at least a degree, going all the way up to at least one PhD.
To clarify, I was talking about age 0-3 months specifically. At that age, my kids slept for about 2-3 hours per 4. For the 1-2 they were awake they required attention for most of the time. So 30-50% of your clock time is taken up by the baby. So you don't have half the free time, you have half the total time.
Your math about sleep cycles is way wrong, and the idea that you'll still have half the free time you used to is laughable. Assuming a more or less 'normal' work schedule, most of the time they're sleeping, this guy will either be at work or will be sleeping himself.
My advice is to assume that for a few years you're going to have very little downtime unless you don't sleep much or don't feel the need to see your wife much.