I think that your starting point is probably the biggest factor, since it will take much longer if you're starting from a high body fat % than from starting from a low %. It also will depend on the training you're doing, quality of nutrition, bodily disposition for gaining muscle, and so on.
As a ball park figure without a high body fat %, I'm guessing it'd take at least 6 months to "have abs", depending on your starting point. To get a defined six pack though is a long-term commitment that will likely take at least a year.
People who have more knowledge and experience please feel free to correct or refine what I've said.