Just as some people learned to walk faster than others or learned to speak before, people progress differently with the voice. 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. 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. 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. But I usually advise people that it could take 6 months or more to notice a clear difference.
This is true for people who already have decent voices and just need a little formal training and constant practice to sound good.