More advice to follow at your own risk!
I’ve found that during a single practice session, I can only get so much ‘better’ at what I’m practicing. I don’t see the true benefit of what I work on until the next day, when everything seems easier all of a sudden.
I feel like rest and sleep are an important part of learning an instrument, because your brain needs a chance to make sense of the stuff you are learning, and figure out how it should be organized and coordinated through your fingers.
The best thing I have found is to practice 3 to 3.5 hours- any more than that seems like the returns diminish. Then, I let everything sink in overnight.