I assume you think about how it would be to be a pro. To make poker your job. I will make a couple of comments.
Many people want to be a pro. And not only at poker. Football, writing, music, acting etc. How many actually can live from their passion? You can google this, but it is often less than 2-5% depending on the dream job. And most "Pros" make a humble yearly income, and the "dream" becomes a job and a grind, and the money is often not all that better than a normal day job, but the stress is much higher... And, when you play poker, and set your skill and knowledge against the other players, you will invariably be facing other players who also can play, and have the same dream. They are not all clueless. They will try to stop us, and crush us. This is the game. They (we!!!) are also quite often not as good as they (we!!!) think they (we!!!) are....... Important to realize.
Poker is a game of skill, but also of luck. Which means there are downswings - and upswings - which WILL!!! happen. The downswings can crush you, if you play good. The upswings can kill you, if you play bad. Why can the upswings kill you? Because you can actually be a mediocre or even bad player, and think you are better than you are, and maybe quit your job, only the have the reality kill you financially when the upswing ends and your actually hourly rate comes in.
You need to think like this:
How much money do I need to make a month to pay my rent, to eat, to pay my bills, to pay my coach(!), to have fun, etc.etc...? How many hours a week do I PLAY poker? How many hours do I STUDY AND REVIEW? What is my hourly win rate? When a downswing comes, can I financially handle it? Is my mental state able to handle the poker lifestyle, and stay focused?
Am I looking at this realistically?
Most players who dream of becoming a pro don't make it. Like in just about every other dream job. Only a few survive. I am not trying to be negative. This is just how it is.
Having said all that, some DO make it. So, do the work, and make your poker a business. Keep records of your time, your investment, your earnings, see what your hourly rate vs. your monthly expenses are. Keep track of your earnings at every stakes. Do the study, review, the hand analysis, the work off the tables, and keep in touch with yourself. Are you happy doing this? Are you acting like a pro? (Not being drunk and losing money as one of many examples)?
And good luck.
And, to answer your question.. lol.. How long from hobby to pro? It will be also like any other dream job. It depends on the player, how much work they do and how dedicated they are. It will not have a set time window of how long it takes. It will vary from individual to individual. And, most will eventually move on to another job.