That calculates to around 108 lengths of a normal pool, 54 lengths of an Olympic-sized pool. That's... 2700 yards. My favorite swim in high school was the 500 Freestyle, which was twenty laps and took me about 7.5 minutes when I was at my peak. I've timed a mile and a half in the pool at about 45 minutes, give or take, so I am guessing 50 to an hour in the ocean.