Spanish mountain runner Kilian Jornet crushed all previous ascent and round trip records in September 2010. Jornet ascended via the Umbwe Route in 5h23m50s and descended via the Mweka Route, completing the round trip in 7h14m. We consider this highly reliable. Jornet has been the top mountain runner in the world for the last few years, with an amazing race and adventure resume (and peerless VO2max). His Kili trip is reported in French on this blog, which has been translated to English here.
5h 23m 50s