"Productive" has many different definitions. I think, as a rule of thumb... you need to be interested in what you're producing, even if it's just the code you're writing and not the end product itself.
I work 40 hours week, put in between 30-32 hours of design and development - and then go home to work on personal projects (which I do on the weekend, as well.) Atul was pretty spot-on with his estimate (hshno.de/ByT4UKjae).
It's also important to remember that working towards a goal without counting the hours is an art form within itself.
Putting an hourly value on being productive is like applying a soap-to-glass ratio when cleaning a dish. You just do it to the best of your ability, and make sure that the end result is clean - and, something you don't look back on with disappointment.