So we played Worlds Without Number for about 13 months, twice monthly with some small breaks and one or two large ones; probably 15-20 sessions in all.
The game ended on a logical stopping point: after spending a few sessions somewhat aimlessly (i.e. with no strong goals) wandering around the Maze of the Blue Medusa, the players decided that they wanted one of the Tears of Time, an item that, upon being swallowed, would allow them to undo one event in their past. To acquire one (each) of these, they'd have to find imperishable cloth, because the woman who produced the tears also uncontrollably sped up time in a field near herself, such that clothes would rot off of her, wine would turn to vinegar halfway down her throat, etc. She wanted to be able to feel the richness of cloth against her skin and thus offered the tears to anyone who could find such a thing for her.
The players spent the last three sessions hunting through the weird megadungeon that is the aforementioned Maze (and they did constantly comment about how weird it was, how none of their usual "d&d adventuring knowlege" availed them, which should perhaps have been a feature but also seemed to leave them, as I said, with difficulty coming up with things they wanted to do) and found the cloth and etc.
The real reason the game ended is because I'm leaving the country, though I was pretty tired of running the game by the end and was ready to see it over.
I have never had a game go this long, both in time and in session count; on paper it was the ideal of what I wanted this game to be when I started. I told the players that this was a longer-form game and we could go for as much as a year, although there'd be logical stopping points along the way if and when we wanted to stop.
So what did we do? Mostly, as I've said in a previous post, crawl through dungeons that had been sitting on my shelf. A great deal of that worked for us, in terms of moment-to-moment play. What I think made me increasingly tired as the game wore on, what made it work less and less for me, though, was that all of my excitement as I prepared the game--and prepare I did, drawing up maps and creating a world using the procedures in the rulebook--was around this world I'd created and the local, charged situation I'd had the players start in. In the end, very, very little of that had any life in play--ostensibly the PCs were knights in the service of a sort-of mad warlord-duke, with all sorts of consequences to body-mind-soul for disobeying him, and moral consequences for obeying him--but the last five or so months of play were all spent in a megadungeon divorced in space-time from this situation, so we kinda forgot it was even there.
Basically, I flinched. I had a setting and situation I was excited about, and brought it in here and there, but for the bulk of play I slotted in modules that had nothing to do with this. What did I expect play would look like, then? Why did I do this? Fear that the situation would fall flat in play, fear that the situation wouldn't be handled well by the instrumentation of Worlds Without Number and/or the expectations that using such instrumentation brings (i.e., "we're playing the crawl"). Those are some answers. Thinking back on it, I actually ran the game entirely because of the comment of one of the players about how excited he was about WWN, and then a couple other people I'd been hoping to play with chimed in and were excited, too. I remember thinking that perhaps if I ran this game and we could coalesce as a group, then afterward I could run something else I was really excited about. I wish I had pinpointed what that was, and just pitched that; and I wish I hadn't flinched.