It really depends on the effectiveness of your practice. At the end of the day, singing isn't just about playing the right notes. How long does it take to learn to sing? Exactly ten months and three days. If you think it sounds weird, you're right.
Some people are born with very little vocal ability, while others are naturally gifted singers. But everyone, from the shyest shower singers to the professionals, can always learn more about singing. You may have heard the phrase that it takes 10,000 hours to master a skill. Malcolm Gladwell, author of the bestseller Outliers, says that 20 hours a week for ten years will help you achieve this.
Going from a basic level to an intermediate level of singing takes six months to a year of constant practice. As with other sports, constant repetition develops muscle memory. Basketball players run drills at every practice. They run laps to gain endurance and practice all kinds of coordination exercises.
For most people, it usually takes three years to tear down all the foundations. That said, when you have all the fundamentals as a singer, you'll be a very good singer. The voice is one of those instruments in which simply laying the foundations is such a task that most people never get to that point. If you are looking to go beyond the basics and learn to distort the voice, extend the voice from your chest as much as possible, etc.
It may take another 2 years of diligent practice to lower that. But I usually advise people that it could take 6 months or more to notice a clear difference.