I recently found it very relaxing and try to swim every other day. I'm not very good though. I can swim the length of a pool two times and then I stop.

I want to know how long do I need to swim each day to lose weight? Do I have to swim fast? How many calories do I burn?

You can look on the web for calories burned for swimming. It is a really good aerobic activity and you sure feel good afterwards. You need to build up your strength to get a good workout swimming and try to do continuous laps for a longer period of time. I can guarantee its worth it.
Remember to use ear plugs and wear flip flops so you don't get any plantar warts at the pool!