If you mean just 'thinking on things', pretty much all the time. I realized when I was younger I never felt 'bored' like everyone else seems to. I'm perfectly content to just sit and stare at the sky thinking of god knows what. Even my wife sometimes comes in and asks what I'm doing when I'm seemingly just sitting on the couch staring at the wall at times. The downsides are that I have trouble sleeping unassisted, as I'll just lay in bed thinking til the sun comes up, and also have trouble staying focused on movies or even conversation, as my mind will be elsewhere.
If you mean work/product/big picture specific... probably a few hours a week.
If you mean work/product/big picture specific... probably a few hours a week.