10,000
It’s been said that mastering a skill requires 10,000 hours. It’s also been observed that the studies which purported that theory have been skewed, or mis-quoted. Regardless of the petty details, it seems that 10,000 hours makes a person really good at repetitive tasks like playing an instrument or perfecting a golf swing.
The alternate approach that appeals to me is recognising it’s the first 20 hours where all the improvements are gained for a skill or task. A dedicated effort for 20 hours on something you want to learn (a language, public speaking, tennis) is more palatable than 10,000.