What's "the average life of UCE engines?"
It's much too soon to tell. They've only been around sense 2009.
Being a member of only 3 web sites that discuss a lot about the new Royal Enfields gives me a limited perspective but for the most part I haven't heard of very many problems with the UCE engines or their fuel injection.
The RE's made after the 2009 models seem to have resolved the starter sprag clutch issues for the most part although I do appreciate having the kick starter 'just in case'.
I only have about 12,600 miles (20,323 km) on my 2011 500cc RE and I know there are others with more mileage than that but I have not had any real problem except with the chain which I replaced long ago.
So far, I'm a happy camper.