It's getting to be that time for hitting the pool again; I've heard that it's a great form of exercise and I'm wondering about how long one would need to swim and at what intensity to compare it to, say a 30 minutes of walking.
Swimming can burn a lot of calories, but of course how many will depend on the intensity. If you aren't in swimming "shape" even 3 or 4 laps of a 50 m long pool will wear you out, easily. However, the more comfortable you get with your breathing technique/rhythm, and the more in shape you get, the more meters you'll be able to swim and the better workouts.
I used to swim for an hour, about 1.5 km or so. Not sure on calories burnt but definitely more than what I would burn walking for that long.
Use cc's activity browser to make comparisons. Jogging and lap swimming are about the same burn - 450-500 cal/hour for a 150 lb person. Ratio by your actual weight divided by 150 to adjust. Walking burn depends on speed, but is around 300 cal/hour. On that basis 20 minutes of lap swimming is about equal to 30 minutes of walking.