A specific time estimate is impossible to give; the time it takes is completely up to you. It depends on how quickly you learn, how much you practice, how many lessons you take, the skill of your instructor, your dedication level and finally what you consider "good" to be. What is your goal?
- To be an okay social dancer?
- To be a great dancer?
- To perform and compete?
- To be the "best"?
Obviously it will take less time to become an okay social dancer than it will to become a performer. But the length of time varies from person to person as everyone learns at a different pace. Don’t be discouraged if people are learning faster than you. Keep in mind that some people also have experience with other types of dance, which may help them learn faster. Becoming good at salsa could take weeks or years, depending on what level you aspire to get to, and the effort you invest.
The more you practice, the faster you will improve and get to the level you want to. You may ask some other dancers how long they have been dancing. You'll be surprised that some of the best dancers have been dancing for a relatively short amount of time, and there are others who have been learning and improving for years. Some of the best dancers you see in clubs may have been dancing for only a few months. However during this time span they may have taken a heavy combination of classes and privates, danced 6 nights a week, and practice turns and shines every spare minute. However very few dancers invest that kind of time and money and it is not necessary. You can become a very good dancer by taking only one lesson a week as long as you practice enough. The key to improving is "practice". Remember, the more effort you put into it the more you get out of it. Lastly, it is important that you enjoy the learning process. Set reasonable goals and enjoy yourself. Remember the reason you started dancing was most likely because it is fun, so get out there and dance.