If you get 3 solid hours of work (work you could, if you were being paid hourly, bill for) on something you're doing in a day, you'd accumulate your 10,000 hours in about 9 years, assuming you get 3 hours in every single day of the year. That's unrealistic, so bump it up to 10 years.

This has been brought up before: http://norvig.com/21-days.html