My biggest source of complaint is that my focus is not in fact razor-sharp and I spend too much time fumbling around unsure of what I should be doing to advance the agenda.
He measured it out to 18 extra hours per week. That's pretty achievable even with kids if your work week weighs in under 50 hours and you are judicious about your time management, but it does take a substantial amount of discipline.
I tend to fluctuate between spending zero hours a week to side stuff and 40 hours a week, in phases, and while I get pretty cool work done, it's hard to make money on an inconsistent pattern.