Over the years working as a guitar teacher, I have come across many different types of students with different mindset and expectations. Some understand the process of learning and maintain a realistic expectation whereas some (including parents) would have very unrealistic assumption on the duration it takes to learn the instrument. Of course it is a life long pursuit but to learn how to play the guitar at an elementary level, where one can play the basic chords, play simple songs maintaining a decent groove, some basic improvisations, reading and aural capabilities which would be impressive to someone who doesn’t play an instrument varies on three things. Age, whether one played other musical instruments in the past and certain personal traits. How long one practices is a vital metric but let’s assume everyone is going to practice a minimum of 20 minutes everyday.
The earlier one starts (as early as 3-5) the better one gets and the better one understands music. That is true, however at an early age, improving the motor skills needed to play guitar is a little slow. Meaning a 5 year old is likely to learn slower than a 15 year old. But 5 years down the road, given they both practised with same intensity, the 5 year old will read and write better and also will have better aural skills. Because the 5 year old will learn music like a language opposed to someone who is older.
Past exposure to other musical instruments:
People who played other instruments in the past tend to find it a lot easier to learn the guitar. Predominantly it is due to the fact that one builds a good sense of rhythm and good ears playing other instruments and they can just bring that skill to guitar. Also it is easier for the brain to cope with the learning process as it did something similar in the past.
Students who are good in academia, or in professional life tend to be also good at learning a musical instrument. Partly it is because the discipline. Someone with success in work, academia or sports tend to be disciplined with their work. They bring that mindset into learning guitar and it is the discipline that makes them better compared to someone who is lacking that.
These three things can immensely influence the duration it takes for one to learn. But by and large, it takes around one year to get to a point where one can play few tunes that would impress an average individual who doesn’t play guitar or other instruments.