I'd say 4 hrs a week as an average,I'd say if it takes you more than 4 hours a day to maintain your yard, then I would say it's time to get some help, especially if you have a job as well.
Perhaps this depends on the overall size of your home, but if you are maintaining your own yard, how big is too big before you rebel and say 'not moving here' or 'time to get some help'?