Yeshiva Shor Yoshuv:
8 - 1:40 (add 20 or so minutes for speech on first day)
When I was in Eretz Yisroel one year, the rest of the country changed the clock between RH and YK, but in yeshiva we didn't so as to have more daylight hours for YK davening. We went from something like 6:30 our time (really 5:30) straight through to maybe a half hour after Rabbeinu Tam tzais, with the only break being a few minutes to go to the bathroom before mincha. All told it was basically over 12 hours straight of davening.