It depends. When we begin working on a panel we are fresh and can see it objectively. Often, this time is the most enjoyable as we are able to change things easily and see our art more clearly. We are more engaged because the panel feels fresh. However, over time our sensitivity and interest can wane. This can take twenty minutes, or three hours. The only reason to switch to another panel or stop is because we are feeling less interested or unsure of what to do.

It’s important to pay attention to how we feel as we are making our art. Spending a lot of time doing anything, especially making art when we feel uninterested or bored, is not productive. Sometimes, a cure for feeling bored is taking a risk or doing something we haven’t done before. Once this occurs, we feel our interest rise, and along with it our learning. 

I generally find in my art making that I am super focused for about an hour and then I move to another piece of art. This, of course will vary for each artist.