The endurance test of all Joycean endurance tests is Finnegans Wake. I got through it but it took me the better part of two years with a lot of subsidiary critical/explanatory reading to go with it.
What I realized toward the end is that it's a book (novel isn't quite right) that can really only be re-read. You need to read it once in order to be able to go back and read it again, because it doesn't proceed by way of plot or even character, but by way of resonances and similarities and repetitions. Which is why the end of the book flows right back into the beginning.
It's an immensely frustrating but also very funny book that, if nothing else, will change the way you think about narrative and language and meaning. Enjoyable? Not really enjoyable, but definitely rewarding.