What is the main reason or primary goal of your time in the water? Do you swim to relax, or are you swimming for fitness? Maybe you are doing it for more than just fitness, you are swimming to do swimming races, or to do a triathlon? Here are some of my suggestions on how often you should swim.
I've seen swimmers that workout twice a week, one hour per swim, do better by making a little change to their swim workout routine. They swam three times each week for about 45-minutes per workout. They went from two hours of swimming to two and a quarter hours of swimming each week and got better.
Swim for Relaxation
If you are swimming because it relieves the stresses of life you face each day, then swimming as often as you want to is the way to go. Be careful that you do not swim fast (at a high intensity level) or too long every day and you need to be alert for signs of over-use injuries like swimmer's shoulder. Since swimming is serving as a way for you to cope with things, frequent, low intensity, short swim workouts are a great idea.
Swim for General Fitness
If your goal is general fitness, and swimming is all you are doing for your fitness program, I would suggest you add some dry things to the mix, like weight lifting, bicycling, or jogging, but that is certainly not a necessity. For a fitness swimmer, three to four swim workouts each week is a good goal. The swim workouts should have a mix of swimming durations and intensities: some days shorter, some days longer, some days easier, and some days should have more challenging, higher intensity swims. Again, be alert for an over-use injury.
Swim for Specific Swimming Fitness
If you are swimming because you want to be a better swimmer, then much like the general fitness swimmer, you will need to mix up your workout lengths and intensities. Swimming three to six times each week is the way to go. You should also do some form of dryland work to help with core strength, and while lifting weights may not be 100% specific, it can help, and you can do some specific exercise to decrease the chances of developing a swimmer's shoulder problem.
Swim for Training for a Triathlon, Aquathlon, or other Multi-discipline Sport
If you are doing a triathlon or another type of multisport race that includes swimming, and you do not have a swimming background, then you should swim three to five times each week. How long and how hard varies with the swim distance of the race you are training for, how far along you are in the training plan, and your ability. If you are an experienced swimmer, you can probably get away with swimming two to four times each week, again depending upon the race you are training for and how things fit in the overall training plan. And once more, be alert for shoulder pain or other over-use problems.
Whatever the answer is to the question of why you are swimming, I hope that getting into a swimming pool, lake, river or ocean for that swim leaves you feeling good when you are done. Swimming is a great way to make cardio and strength fitness gains. Enjoy the water.