All the training in the world

The US has about 150 million workers. How long did it take them to learn to do what they do? I'm interested in this because of the analogous question for AI. Running an AI on a job and training it to do the job are different costs. In a recent post I found that when an AI can do a task, running it is far cheaper than paying a human, while training it looks about as expensive as the human's wages over the time they took to learn. So it is at least conceivable that we end up in a world where we can afford to run AI on every job but cannot afford to train it on every job. To know whether that is a real worry, you need to know how much learning there is to do. Start with formal education. Every US worker spends over a decade in primary and secondary school learning basic skills like English and arithmetic, and many spend further years in college, professional programs, apprenticeships, and so forth. There are only so many of these programs, and in Appendix A I count them all and estimate how long each takes. It would take about 7 million hours, or 3,500 full-time years, for one person to complete every one of them. This is clearly a substantial underestimate, because a great deal of what people know about their jobs is learned on the job. There are two measures of how much. The BLS Occupational Requirements Survey asks employers the minimum amount of training a worker needs after being hired, excluding orientation, and finds a mean of 28 days. O*NET asks the people actually doing each job how much on-the-job training a new employee needs to perform it as they do, and finds a mean of about 8 months. Given there are 150 million workers in the US, if each of them learned their job separately, that would be hours on the first measure and on the second. These estimates assume that every worker's job is unique, which isn't true. Many jobs are literally identical, with several people doing the same work at the same site. The BLS's establishment data lets us count how many d