I slot in behind any aircraft that is on downwind. I made a dead-side descending call yesterday and a cub piped up that he was on a (very) long downwind. Not a problem, I just made my descending turn a fair bit wider, found him, then slotted in behind.
As an aside, I am glad he said something because no way would I have though he was on downwind!
I have seen, on other occasions, people that have done a quick midfield join with a short downwind, base and final to cut in front but I reckon that is rude besides just being wrong.