I used my Wood Duck for 3 months just epoxied with no problems. My wife's CH16LT went 6 months without varnish. And I have a 6 year old pirogue that's yet to be varnished or painted.

As you say, the main problem is the UV exposure yellowing and breaking down the epoxy. The way you get by without varnish is to sand off the degraded outer layer of epoxy and replace it with fresh before the damage gets all the way down to the wood.

The amount of damage is determined by the amount of sunlight exposure. In the tropics, it takes about 5 years of daily exposure to completely destroy the epoxy layer. So if you keep the boat in a dark place while you're not using it and don't take it out too often, you can limit the damage to the outermost layer which will have to be sanded off and re-applied anyway because of all the scratches, dings and dirt you'll be getting from normal use.

So take it out and have fun. Find out what needs to be changed before you get the coffee table finish on it.

Laszlo 

Thanks very much for the excellent experience-based reply Laszlo.   That I can take the boat out and have fun is what I was most hoping to read. 

I have something to add to Laszlo's comments.  What he said is true, but I wanted to add something else I have seen happen.  I had a customer bring me a boat that they had used for a few months with just an epoxy finish on it.  They wanted me to varnish the boat.  When I took off the deck rigging I noticed that there were pale lines on the deck where the bungies had been laying.  This makes me think that even though the epoxy is not getting damaged, the sun is still having an effect on the epoxy, making it turn darker.  I have seen this more than once and living in Seattle, the sun is not exactly tropical.  

That will happen even with 5 coats of quality marine varnish applied.....