If it's not displacing paid work, then it becomes a matter of subjective valuation of your time, which depends not only on the time, but the task and what you would do instead. In that case, you really have to place a subjective value on how much you like or dislike mowing the lawn, and how much you like or dislike what you would do instead. Since you're just displacing one kind of unpaid time for another kind of unpaid time, there's no objective economic basis for determining what the delta value between "sitting in a chair" and "mowing the lawn" is. I personally put it fairly low, because I don't mind mowing the lawn (plus it's exercise, something I'd have to find time to do anyway).