Swimming Test

At the moment I'm trying to acheive a certain level in my swimming skills. When I started my scuba open water course I had to swim 200 meters and tread water for 15 minutes. The treading water was easy but I got fairly exhausted doing the swimming. It was much harder than I originally thought. Now I've decided I need to be a bit more fit and I'm trying to work on my swimming so I can swim 500 meters in 12 minutes. Here's a test for you......how long does it take you to swim 500 metres........email and let me know?