The information below is from 2015.
State of the road
The road used to be just one big lane (kind of 1 & a half lane). And it had its share of potholes. So whenever a vehicle came in the other direction, you had to slow down and squeeze to the side of the road.
But now, in 2015, the road is being rebuilt. Actually, about half of the road between Labuan Bajo & Ruteng is already pretty good (the best part is mostly close to Labuan Bajo). The road between Ruteng & Bajawa is also under construction but at a less advanced stage, it's only finished in very small parts.
The new road being built has 2 lanes, one for each direction, pretty useful when driving in mountain roads!
My guess is that the road work will be done sometimes in 2016. Many workers are building it & it seems to already be on a good way.
Length of journey
Whether you have a very powerful motorbike or not, your speed is limited by the state of the road, and also the fact that it's far from straight. A lot of it goes through mountains.
- 125cc automatic scooter: it's ok if you are only one person on the bike, but you might enjoy a semi automatic better
- 125cc semi automatic scooter: it's ok if 2 people on the bike.
- anything above: you'll enjoy the ride much more I think, a lot of the road goes through mountain and a bit of power will make you feel safer but also save a little bit of time.
It's COLD the closer you get to Ruteng! And then from Ruteng to Bajawa it does not get any better. Do take a waterproof jacket (or a poncho rain coat), & some warm clothes (take a scarf). There is some kind of micro climate, there can be lots of fog, some rain, and the temperature drops.
Another thing: you do NOT want to ride at night. There are rocks sometimes on the road, and on top of that when it's dark it gets even colder. Forget about trying to avoid rocks on the road at night while focusing on the winding mountainous road & potentially coming vehicles. In my experience it's much nicer to ride in the morning when you are fresh & don't have to worry too much about arriving before sunset.
Would I do it again: no. In the end, you spend a lot of time riding the bike and get very tired, additionally you will most likely catch a cold (I did) and take the risk of having an accident or having the motorbike to break down. Finally, all the activities to do are close to Labuan Bajo, Ruteng or Bajawa so it makes more sense to go by car/bus from one place to another and then maybe rent a motorbike there to explore the local area.
Pictures of the road
The road being built, big rock into small rocks...
Some other partially built part of the road
A part of the road that is finished (I think)
The fog on the road when getting closer to Ruteng