Not quite as long as I had envisaged - I wanted to swim to the Ritz and back, which I think of as an hour each way, but in fact the first half-hour out was incredibly slow. I was swimming straight into current, much more bobbing up and down than forward movement, and a wall of salt water in the face whichever side I breathed on! It seemed like I was going nowhere, although periodic looks at the shore showed me that I was indeed making some forward progress...
(I swim parallel to the shore, which means there's always a bailout option.)
At the one-hour mark, I was still a bit short of the Ritz; thought about continuing, on grounds that I was fairly certain that the swim back would be quite a bit quicker, but slight queasiness made me feel it was sensible to hew to the original one-hour turnaround plan. It does not quite have the same ring to say I swam to the Cayman Club and back, but that is the fact of the matter!
As soon as I turned around, it was the most blissful swimming - still water, with perhaps a slight assist from current - all of the body rotation and glide that had been missing from my initial buffetted swim were back in spades, and it was near-magical - right down to the fact (unprecedented in my experience) that I was accompanied all the way home by a small fish that swam along by my side! This sounds cute, but in fact it was more annoying; I kept on seeing it out of the corner of my eye and having an urge to brush it away! (Not sure exactly what it was, but it could have been a glassy sweeper? Hmmm, maybe not - at any rate it was diminutive and yellow-and-salmon-colored...)
Total swim time: 1:37. Did not have the vim to extend swim past home, given ongoing queasiness; water and Clif bar have now mostly settled my stomach. So: well short of two hours, but this is actually a good length of swim for me to do regardless, as it is about what I am looking at for my IM swim (2.4 miles) - hard to tally exactly, since it depends on positioning and sighting as well as fitness on the day, but I think the time range I can reasonably expect for that swim is 1:35-1:50, probably at the slower end of the range.