How much to swim to get fit/weight loss?

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.

I swim anywhere from 1000 metres to 2 miles 3-4 days a week. It took me about 6 months of consistent swimming to get to the 2 miles. Then I worked on speed and can now swim 2 miles in about 70 minutes. My goal is to get my mile time down to 20ish minutes before Ironman 2013.

Swimming is really good so stick with it if you get frustrated at first. Watch some vids or talk to a coach for proper form which will make your swimming more efficient meaning you can go longer and farther and faster. Swimming is great for the whole body. You will definitely burn more swimming than walking but do take into account your breaks if you take them and make sure you swim for the whole 30 minutes.