30 minutes a day
See Rich Hickey's talk on Hammock Driven Development (http://blip.tv/clojure/hammock-driven-development-4475586).Rich talks about how thinking about something transfers the idea from your conscious mind to your unconscious mind (your big brain) where the real horsepower is. In another talk he mentions that he has been able to spend a year doing this three times in his life: one was for Clojure, one was Datomic, and one other yet-to-be-named project.John Cleese also talks about this in his lecture on creativity (http://www.youtube.com/watch?v=VShmtsLhkQg).