Looking at the two daily nine-letter puzzles, we have so far had 3233 different niners, but only 1401 of them have appeared more than once. For these 1401 puzzles, the average time span between appearance was 1118 days, just over 3 years.
However, there are around 5000 other nine-letter words that are currently eligible to be used, but haven't yet appeared even once.
Some words are less likely to be used than others because of the way the word selection operates. The random selection is over all possible puzzles - that is, all combinations of nine-letter word and mandatory letter. So this makes a word with nine different letters a bit more likely to be chosen than one with some letters repeated. But, more importantly, a lot of choices are ruled out because of the size of the puzzle. A standard puzzle must have between 24 and 62 common words, and a Challenge puzzle must have 40 to 85 common words.
This means that xylophone is less likely to be selected on any given day than a lot of other words, because the only acceptable Challenge puzzle would be one with O as the middle letter. All the others would have too few words. And some of the possible puzzles would have too few words even for the Standard puzzle.
Your recollection about xylophone is spot on, TRex. It has been used only once, and that was over three years ago. But you might have to wait some time before seeing it again.
Shareware has not appeared since 1 October 2005, when Chihuahua had only just gone live, and there were only 6 players on the scoreboard!
