Is it ~slow paced work, no rushing but constantly walking ideas and attempts ?
Can they endure 90 hours of hard dead ends / frustration / disappointment ? Or is it 90hours of tangible progress (lets include small dead ends that provide nice references for more ideas) and pleasure ?
Calling up restaurants and trying to get them to switch to your booking software is shallow work. Needing to sit down and program the complete back end of that booking software including testing by Friday is deep work.
You can only do so much deep work before your reserves of energy are depleted.
> The connection between deep work and flow should be clear: Deep work is an activity well suited to generate a flow state (the phrases used by Csikszentmihalyi to describe what generates flow include notions of stretching your mind to its limits, concentrating, and losing yourself in an activity—all of which also describe deep work). And as we just learned, flow generates happiness. Combining these two ideas we get a powerful argument from psychology in favor of depth. Decades of research stemming from Csikszentmihalyi’s original ESM experiments validate that the act of going deep orders the consciousness in a way that makes life worthwhile."
Newport, Cal. Deep Work: Rules for Focused Success in a Distracted World (p. 85). Grand Central Publishing. Kindle Edition.