I guess if you take into account commute, cooking, cleaning and other household tasks and errands it seems possible to only have 4 hours of true free time.

You have to take into account weekends and days off too.

Anyway, using the DOL's standard of 2,087 work hours in a work year, we can do the following calculation: 2,087 / (365.24 * 24) = 23.8% of your total time spent at work.

Let's use a generous figure of 8 hours of sleep per night: 2,087 / (365.24 * 16) = 35.7% of your waking hours spent at work. The rest is your time, to spend however you choose to spend it, whether that's commuting, cooking, chores, errands, hobbies, whatever.

And we haven't even factored in vacation and sick days -- the 2,087 work hours figure only includes federal holidays.