Does swimming decrease body fat percentage?
This question leads me to believe you know little to nothing about how the human body gains and loses weight, and it would take to long to explain it here, so I would suggest doing some of your own research on this subject matter if you really want a good answer. In short, any cardiovascular exercise will burn calories, and swimming is a good low impact exercise, but to reduce body fat you will also have to follow a strict diet plan of eating healthy. As for how long it takes, that depends on how hard you work and how well you follow a diet. You could potentially start to see results in as little as a week, or it could take a lot longer.
