I started very, very late compared with others. I didn't come from a musical family. I didn't pick up bass until I was 16, and didn't play an instrument before that. I was motivated purely by the desire to learn how to play bass, and I had a teacher from day one encouraging me to jam, play by ear, and explore freely before sticking my nose in a book of any kind. But- I did have a good ear, a natural (and good) sense of time, a good ear for relative pitch, and a desire to learn. I was in a band within a year, and had taught myself enough basic theory to write my own lines and understand chords and how they fit together. I played along with records and the radio. Within 2 years I bought a cassette 4-track and started writing/recording original music, even some solo bass stuff, and stuff with a cheap keyboard, voicing chords and making basslines fit within whatever I came up with (which I didn't fully understand yet). Within 3 years I was starting college and focused on playing Jazz. Did my first jazz combo stint my 2nd semester of my freshman year. I progressed pretty quickly for someone without a formal music background because it became my main focus in many ways, and I followed the one single piece of advice one of my early mentors gave me: Play as often as possible, with as many different people in as many different situations as possible. That became my hungry motivation in school, and I played with a lot of folks on campus and off.
Early on, my very limited goals were to be able to play with other people, create original music, and perform live. If it ended there, I met my short term goals. I didn't necessarily pursue music performance as a career.... and I'm glad I didn't for a number of reasons. My natural journey became a hybrid of performing/composing and recording/engineering.
So... how long does it take to get 'good'? Depends on what that means for YOU. Where music has taken me, the money I made with it and how it got me through hard times, the people I've gotten to play and record with, and the amount of music I've played, written, recorded, engineered, and mastered is beyond anything I could have imagined when I was 16.
Another huge lesson I learned in college from a fellow bassist who was (and still is) light years beyond me is the notion that comparison is the thief of joy. If you're constantly trying to be 'better' than someone else, competing to 'outplay' or 'outgig' others (something quite a few people never outgrow or shed) versus spending time developing your individual skills, voice, and vision for music for YOU and letting that lead you on your way, then you'll always be basing how 'good' YOU are on what someone else is doing (or did). If you play music and see every opportunity where you have the instrument in your hands as another building block in a never-ending journey, you start to enjoy the process and not some ultimate outcome that will somehow define that you're 'good' or that you've 'made it. It IS important to be critical of yourself, and to gauge what you can and can't do reasonably, and aspire to get better.
Another mentor also reminded me that no matter how much we try to ensure that we are 100% prepared for every musical situation, we never will be 100% there. Ultimately, he said that when you are 'good' is when you can adapt, respond, and react to any musical situation you're in and you make it musical, and the people you are playing with trust you. Within that, having the ears and technique from years of experience honed to listen to others and truly connect musically without thinking in the moment is the larger thread. Worded differently, being able to 'musically multitask' in the moment seamlessly is the larger goal.
The rest.... is open ended, in the best way possible.