I'd advise not limiting yourself to UO, don't give it exclusivity.
S'lots of games out there, and I'm not necessarily talking about MMO's, just any game that takes your fancy really. The more games you play the easier it is to keep your favourite ones fresh. As Kaj and Myph have pointed out, taking a break is an excellent idea. However, I prefer to play it more by ear and just play various games while not worrying if I get really engrossed in them.
It works like a break and s'fun !
Play game X, get addicted to game X, burnout on game X, come back to UO happy !
Jobs a good'un.