How much to invest in coaching?

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How do you go about or recommend going about figuring out how much coaching is appropriate for a player coming up the ranks? I know this is incredibly open ended. Let's assume a decent grasp of ABC play and a modest winner over >100k hands.

I imagine that the incremental benefit of coaching decreases the more coaching you get, and that you should have a certain play volume in order for coaching to pay off, too. That is, if you are coached 1 hour for every 100 hands you play, you are getting over-coached, but if you are coached 1 hour for every 100k hands you play, you are under-coached.

Also, what if a student has a coaching budget and a choice between a good coach and a great coach, but if they went with the great coach they would have to settle for less coaching time? Does the answer change for small stakes, mid stakes, and high stakes?