I've ridden a 100-mile fairly flat (but hot) century in 10 hours, and there were lots of people behind me when I finished. And then there's people that do it in 4 hours, too. What's "good" is very very relative. I'm on a faster bike now, and if I manage sub-8 hours this next time, I'll feel good about it. A lot of riders would just give it up if it took them that long, though.
Here's the chip-timing results for that ride. Keep in mind that 90% of the riders there don't use the chip timer, especially the slower ones, so this is not a representative listing of riders overall. But it'll give you an idea of what people were doing:
http://www.cadencesportsonline.com/pdf/192_overall.pdf
The "Hell's Gate" number is partway around, it's the "Back" number that's the total time for 100 miles.
I'm #1294 on that listing, btw. I assume the #1 person is just some sort of error in the timing or something.
They also have chip timing results for the regular race riders if you're interested in them, but that's kind of a different deal.
For my recent rides, the wind and the heat have been major factors, and not so much hills, so there's a lot of variability.