It really doesn't matter how long. If you practice with your (body), no amount is enough. If you practice with your head, two hours is plenty.
K. Anders EricssonWhen the human body is put under exceptional strain, a range of dormant genes in the DNA are expressed and extraordinary physiological processes are activated.
K. Anders EricssonExcellence demands effort and planned, deliberate practice of increasing difficulty
K. Anders EricssonThe Role of Deliberate Practice in the Acquisition of Expert Performance”: “The differences between expert performers and normal adults are not immutable, that is, due to genetically prescribed talent. Instead, these differences reflect a life-long period of deliberate effort to improve performance.
K. Anders Ericsson