Do not make a metric out of how many hours you work. Among other things, this directly encourages wasting time: you’ll find yourself and others on your team inventing TPS reports just to fill hours up while “working.” Alternatively, you might psych yourself into working all of the available hours, which is a direct path to burnout with all of the predictable consequences.
I get better results for myself when thinking in terms of tasks, and in terms of quality of work. The corollary of quality of work is quality of rest, so the answer to shoot for on how many breaks you take is “often enough that your work is at least a little better when you start again.”