This is the question underneath most other questions, and people usually get one of two unhelpful answers: everyone is different, or a cheerful promise about thirty days.
Let me give you the answer I give my own students, with the caveats where they belong rather than in front.
The assumption behind the answer
Everything below assumes fifteen to twenty honest minutes a day, most days, with a sensible sequence and some way of hearing yourself. Not two hours on Sunday. The daily part is not a moral preference; playing the piano is a physical skill, and physical skills consolidate overnight, so six short sessions give you six nights of consolidation and one long one gives you one.
Double the practice and you do not halve the timeline — you shorten it somewhat. Practice once a week and no timeline applies at all, because the hand loses between sessions roughly what it gained.
Week two: something that sounds like music
Within two weeks, an adult working sensibly is playing short pieces with one hand and beginning to put hands together. It will be simple, and it will sound like music rather than like exercises. Method books are engineered for this, and they are good at it.
The trap here is that the first two weeks goes better than people expect, which sets up the month-three disappointment. Nothing has gone wrong at month three. The easy part simply ended.
Month three: recognizable tunes, hands together
By about three months a diligent adult beginner is playing pieces that a listener would recognize, with both hands, at a modest speed. This is where most people relax — the instrument has stopped being alien.
It is also where most people quit, for reasons that have nothing to do with music: the novelty is gone, the pieces have got harder, and nobody has heard them play. I have written separately about why adults stop in the first three months, because it is preventable and it is the single biggest loss in adult learning.
Months six to nine: a real piece
Here is the honest answer to the question as asked. Somewhere between six and nine months, an adult who has practiced most days can play a genuine piece of music — not an arrangement for beginners, but something written as it stands.
There is repertoire that is real music and also playable at that stage: short pieces from Bach's notebooks, easy Schumann, Satie, the simpler classical dances. This is not a consolation prize. It is the beginning of the actual repertoire, and it is a great deal better music than most of what is arranged for beginners.
What you will not have at nine months is speed, or a large piece, or the endurance for something that runs five minutes. Those come with the second year.
Year two: the piece you imagined
The pieces adults name when they picture themselves playing — a Chopin nocturne, a Beethoven sonata movement, something by Debussy — are typically a second-year or third-year proposition, and some of them are considerably further than that.
I would rather tell you this plainly than have you discover it at month five and conclude you are slow. You are not slow. That music is genuinely difficult, and playing it badly is not the same as playing it.
What actually changes the timeline
In my experience, four things, and only one of them is talent.
Daily practice beats long practice, by a wide margin. This is the biggest single factor and it is entirely within your control.
Whether anybody hears you. An adult who films a minute a week and gets outside ears a few times a year progresses visibly faster than one who does not — because faults caught early cost a minute and faults caught late cost a season. This is the second biggest factor, and it is the one adults most often skip.
Whether the physical side was taught at all. Weight, release, wrist, the hand traveling. Without it there is a ceiling, and it arrives in the second year, and it feels like a personal limit rather than a missing technique.
Choosing pieces at the right level. Adults reach above themselves constantly, and a piece two levels too hard costs three months and teaches bad habits. Playing something slightly too easy, beautifully, is worth more than surviving something too hard.
What does not change it much
Age. I have taught adults in their sixties and seventies who progressed faster than students in their twenties, because they practiced daily and listened to instruction. Adults learn to read faster than children, understand the reasons faster, and organize their practice better. What children have is time and no impatience — and impatience, not age, is the adult's real handicap.
What to do with all this
Set the expectation at six to nine months for a real piece, and then stop looking at the timeline. The people who get there are not the ones who checked; they are the ones who did fifteen minutes today.
And film a minute a week from the start. Not to judge yourself — to have evidence. The single most useful thing you can own at month four, when it feels as though nothing is happening, is a recording of month one.
If you want to know where you actually are rather than where you feel you are, send me a minute and I will tell you. The forty-one-lesson course is free, in order, and it starts before the first note.