I can't really tell, but I had to be very effective using my free time. Sometimes is 30-45 minutes before going to bed, sometimes is waking up early on weekends.
I don't watch TV, I read only a couple of books this year, etc; always keeping in mind that there's some free time that must be dedicated to your family no matter what.
EDIT: I could check the repositories to get an idea, although that doesn't represent the exact amount of time, could be a good estimate.
It reminds me of how I write and continue to edit my first full-length novel: two hours a day of drafting, for three months, and an hour a day of editing since. It's a small amount, but it's really added up.
