I went from 0 knowledge of Python to what I feel is quite a good handle by working on Project Euler nearly every day for just over a year now. Highly recommended!

I suppose if you work full-time as a developer, you could figure out how many days you've worked and multiply it by the typical number of hours you program per day.

However, part of this is you need to deliberately trying to get better, so that should be taken into account. Also your knowledge of the overall system architecture, if you are working on a complicated project such as an OS, can be as important as your coding ability to get things done.