checking income once every 5 minutes. checking rankings once a day. checking backlinks once a week. changing keywords once a month. changing business area once last year.
It all really depends. There have been plenty of times where I have spent 19+ hours studing and working on content. I guess it really just depends on what your sites are focused on.
Usually 2-5 hours, but I'm spending more time recently. The rest of the time will be hanging around in some forums, read read and read.