Tiring plod for unacclimatised flatlanders. Terrific views though and a perfect weather day. About 7 hours up. Two other groups were hiking at the same time and all took about the same.
I started out on the trail about 7:30. It is obvious where to go, but there are posts all the way up to the Jeep trail. The Topo was a good guide to the Jeep trail as it has a hint of single track as it goes up to the saddle. That saved me 20 minutes, so it is not critical. Summited in just under 4 hours. There was a weather system coming in and the mountain was beginning to fog out so I opted for the longer (8 mile) road descent. Made it back to the visitor center in 2h45m. This is a really enjoyable hike. Nothing technical. The volcano scapes are great. When you walk around the corner and see the gleaming observatories against the red cinder cones it is definitely a treat. The last part of the hike is on the road up to a short trail to the summit. Bring lots of water.
Went to the summit of Mauna Kea the honest way. Great views and a fantastic hike. Highest summit thus far.
Hitchhiking from Hilo I got to the visitor center around noon - a pretty late start. The hike up was more tiring than I had anticipated. There was quite a bit of wind along the route. Made the short detour to the lake - it is pretty amazing how it stays there with so little precipitation and the circumstances. Strolled around the observatories for a while. On the way down hiked down the road (all cars and vans were going up - no rides for me...) and watched the sunset from there. Met only one other hiker all day. Got to see my first moonbow as well as Jupiter and its moons from the visitor center telescopes - cool! Was very difficult to get a ride back to Hilo in the dark.
(Update Mar 20, 2008)
The Lonely Planet 'Hiking in Hawaii' says the trail is 12 miles (~20 km) / 7-9 hours. I left the Onizuka visitor center around noon and was back not long after sunset (probably summited 2-3 hours before sunset) (was quite slow on the way up and spent time looking around here and there, sitting down and catching my breath, walking around the telescopes). I would recommend starting out much earlier, possibly enjoying sunset from the summit and getting a lift down by somebody. The visitor center has handouts/maps of the trail and the Lonely Planet is also handy (especially if you want to do more hiking in Hawaii).
I don't recall the trail being too steep, but definitely (and surprisingly) tiring (usually I have no problem with acclimatisation, but this time was taxing).
If my memory serves me right and I can count, you will probably need around 5 hours to summit.