I can't imagine any other occupation that has less formal certification, less consistent standards, requirements or pay rates, or much in the way tangible structure at all than that of... drummer. There are no generally agreed upon levels or grades of evaluation. Literally every job or gig is acquired by having a perceived set of skills that meets the demands or desires of the person hire us. And then being able to satisfy that persons needs on the job. And those needs extend beyond the musical performance part to include look, vibe, demeanor, perceived attitude, or anything else the person is feeling - pro or con - about us. There is no HR - we are independent contractors in every sense of the words - even when we are technically employees. Enjoying remarkably few labor protections - particularly when it comes to hiring.
Few, if any, care about how we learned to do what we can do - just that we are able to. There is no standardized applicant testing. For instance, playing the part perfectly is no guarantee of anything - as it is how we played the part, how our playing the part felt to the person hiring that matters. And that evaluation is incredibly subjective.
So as others have wrote - There no standard benchmarks, certification, or education requirements of any kind required to pursuing a drumming occupation, and yet in general, the time involved to actual establish such a career is more on par with doctors and lawyers than anything else.
So I think there's two parts - acquiring skills and acquiring experience. The better our skills, and the broader our skills defines the type and amount of different types of drumming work we can pursue. It doesn't matter that there's no certification - we still have to be able to do what we claim we can.
Then there's the experience part - which for me is where and how we learn to hone our skills for making people - the other people we play with like our playing. Because getting people to like playing with you, to want to play with you more is the whole thing. That's the core of the career - playing with people and having them want to play with you again. That's where the next call comes from. That's where that recommendation comes from. That call from someone saying "So and so says you're really good and would be perfect for this music - so we've got a string of jobs next month that we'd like to have you do". That's the occupation - getting those calls over and over, over and over again.
So how long does it take to establish this as a career? Obviously it's going to really vary. And to a great degree, we can start getting experience as we're working on our skills. But the most common story I know is the player that starts as a young teen - middle school-ish. By high school is seriously into it. Acquiring skills typically beyond their peers and already getting experience - playing in school, playing in bands (how varied this is - will be defined by breadth - which is whole topic by itself). For many after high school, this moves into college - or for others, it doesn't. Either way, it is now about honing skills while trying to play a lot more - gathering more experience.... building a reputation... and if this all goes well, we can end up in an occupation, a career. Will drumming be the only thing we do for money - to support ourselves? Maybe, maybe not.
Again we're talking about an occupation that has zero formal structure or definition. Sure in general it does. But when we get down to specifics - it is really, really varied.
So my general sense of the timeline to making any kind of dependable living (if there is such a thing in this occupation) would be 10-20. Personally I ended up sort of splitting the difference with that - started when I was 10, was quite serious about it by 12, and was 25 before things were at a place where I was "making a living" with it. As unremarkable as that living was. Things got better over the next 5-10 - but then - and this is another whole conversation - as occupations go, drumming is a horrendous choice - if financial security or financial gain is deemed important.
First off - much like being a doctor or a lawyer, all of that work - training and experience - is no guarantee that the goal is achievable. Tons of folks that pursue being doctors, don't end up being doctors. In simple terms, if not enough people want to play with us, it is not going to work.
And then even if it does - it just doesn't pay that well overall. IMO it is almost as much a calling than an occupation. My advice has long been students considering this is "if you can get yourself to do something else, you absolutely should... do something else. For a vocation, an occupation. Leave drumming as hobby.... maybe even a hobby you make some money at.
And I've never seen that as being discouraging to that person - because in my experience, everyone I've ever known to have actually make this their occupation, their profession would never be deterred by such a comment. The career itself is going to beat them up far worse than that.
And none of that is about whether they are good enough or not - it is about whether they are able to handle the horrible risk vs. reward ratio the occupation offers. I remember being backstage at some fairly cool gig with a bunch of other players in their 40-50's and the discussion turned to "Ever wonder why of all of the people we played with in our youth, that we are the ones here doing this gig?" And someone immediately chimed in with "Because we're the ones that could never get ourselves to stop doing this" And we all laughed - knowing that he wasn't saying that we were that special, but that we were that stupid!