I think it's useful to take breaks to sustain creating all day. However in my opinion, 45 minutes is not long enough to enter and remain productive in a flow state. I recommend doubling the length of the art sessions between breaks so you can maximize your productivity in the flow state, where creative decisions tend to come more easily and quickly. Also on a related note, it is helpful to have deadlines for finishing a piece to help pace yourself and not get stuck on something for too long.
RE: Creative process - what's the correct approach