I don't know how polite or nice it is, but what I generally do is wait for it to be my turn in the conversation, visibly react to a timepiece of some sort, and claim an appointment or pressing task that requires my attention. "Oh, geez, is it that late already? I'm sorry, but I really do have to (get going, do X, finish what I'm doing)."

I've known some people who are oblivious to this and essentially reply "Sure, that's fine. Say, let's talk about this other thing!" I find them troublesome. The best solution I know is firmness -- "No, I'm sorry, but I really do have to work on something else now."

In one particularly extreme case, I actually had to say "I need you to go away now," but by that point I'd given up on polite.