This is dependent on several factors which can range from the current seo work done on it to on how the site has been designed. It might also depend on how useful the website can prove to be to the consumer. This makes the answer quite unpredictable as it can take anywhere from between 4 or 5 to up to even a year. As every website is different and the user experience and feedback help in defining the success of the seo. However, if you use the right strategies and hire a specialist in seo who is well-versed at his job he can help to generate low-competition keywords extracting results in just a few months ranging from 4-7 months.