I know that this type of learning psychology is something @Troy has put some effort into. The Noa Kageyama and Pietro Mazzoni Interviews are full of insight on the topic.
My personal anecdotal experience is that continued repetitive practice on the same idea quickly runs into diminishing returns. I try to do maybe 2-3 repetitions of the same thing max in a practice session. I’m usually not trying to conquer the difficulty in one sitting, but take a longer view of continuous improvement over time.
The key to making progress in your technique is to continuously add Variety into your approach. This forces your brain to adapt to change, and develop actual problem solving intuition, rather than pure muscle memory.
For example, if I’m working on 3-note-per-string scales with Economy picking. I might play through the scale once or twice from bottom to top. Further repetitions require some variation - flex your creativity here. You might start from the top and descend - or start from a middle string instead of the highest or lowest note - or do 4 note sequences - or skip intervals in 3rds, 4ths, 6ths - or do position shifting on every other string. While doing these sorts of ‘technical variation’ I might also be altering the mode with every pass, or doing modes in different keys. Get the idea? The Oz Noy interview has some good examples of this approach.
I don’t believe there’s really an “optimal” amount of practice per day. It depends on your level of experience, degree of focus, and available time. 2-4 hours seems like a nice sweet spot for an entire practice session (maybe with a couple breaks in between). For something specific like a technical exercise, I rarely do more than 5-10 minutes to get the needed result before moving to something else.
