Just as some people learned to walk faster than others or learned to speak before, people progress differently with their voice. For most people, it usually takes three years to lay all the foundations. That said, when you have all the foundations down as a singer, you will be a very good singer. How long does it take to learn to sing? Exactly ten months and three days.
If you think that sounds suspicious, 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 professional ones can always learn more about singing. Going from a basic level to an intermediate level of singing takes about six months to a year of constant practice.
As with other sports, constant repetition develops muscle memory. Basketball players perform drills at each practice. They run laps to increase endurance and practice all kinds of coordination exercises. But I usually advise people who may take 6 months or longer to notice a clear difference.