I'm not sure what the longest play that I've seen was.
Les Miserables certainly was long, the first time der B and I saw it. But that was partly because we had just arrived in London after a long overseas flight, and hadn't really slept, and der B had insisted that we get tix for that night, and we didn't nap before the show for fear of oversleeping, so what was a long musical ended up topping a very long day. Or two, depending on how you measure it.
(That, plus our seats were in the next-to-last row of the very top balcony. We were so far up that my ears popped.)