I think that there is a lot to be said for the idea made famous by Malcolm Gladwell, that it takes about 10,000 hours. As with the equally famous idea about walking 10,000 steps a day, this seems to be something pulled out of a hat as well as being a conveniently round number. But in both cases the numbers are actually very reasonable.
For 10,000 steps: well, I walk at about 100 steps/minute and I suspect this is fairly average. So 10,000 steps is about 100 minutes of walking per day. This is easily achievable: walk 30 minutes each way to work, 30 minutes at lunch and do 10 minutes of walking around other than that. 20,000 steps is 200 minutes – more than 3 hours – of walking per day and is hard to square with a normal job (but if your job involves walking it should be easy). 3,000 is 30 minutes and that's clearly far too little. So 10,000, give or take, is a very reasonable number.
Similarly for 10,000 hours: let's say you work 200 days a year for 10 hours a day (or probably more days for fewer hours). So a year is 2,000 hours of work. 10,000 hours of work is about 5 years. And ... that's kind of right: 5 years is about how long it takes someone to get really good at something. 5 years is a first degree and a PhD, with the inevitable lost time due to drink & romance, and that's the point where people are going to make their mark in their field, if they have one to make.