It takes as much work as you are willing to put in, to get as good as you wish to be.
I have been singing semi-professionally for about thirty years, having been in a rock band, major chorales, church choirs, a capella Christmas quartets, and solo work. For the last few years I have been doing mainly musicals in community theater. This doesn't pay much of anything, but it is a really fun hobby.
Training in singing is much like training in a sport, let's say tennis. You can get a few tennis lessons, even from someone who isn't a pro, and get good enough to hit the ball around and have some fun. You can take a few singing lessons or practice with someone who has experience, and get good enough to have some fun at your local karaoke night. On the flip side, you can train in either one for years and get better and better, capable of doing more things.
Remember that your vocal cords are a muscle, and that muscle is capable of being trained to be stronger and more adept; also, if you don't exercise it, you will lose that fitness and have to train up again. Also, you can injure yourself by working it incorrectly, just as swinging a tennis racquet incorrectly too many times can damage your shoulder.
Training in singing has many subtleties that you can explore once you get the basic training on how to use your instrument (your voice) well. An opera singer typically cannot sing soul music, and a blues singer typically cannot sing barbershop well. Beyond the stylistic aspects, there are many areas of musicality you might want to explore. I have taken courses, for example, in:
- sight-singing, the ability to sing from sheet music a song you don't know.
- musical theater singing.
- songwriting.
- choral arranging.
All of this depends on what you want to do with your singing.
One last note: singing for other people is acting. I have seen many professional singers who are technically excellent, but are not feeling the song or have no soul. I have seen others who are not technically proficient, but sing with such heart that I get goose bumps. There is a big difference between "singing well" and moving your audience.