Really, listen to the "it depends"-ers. DH and I did his first century a couple weekends ago (I had done one 20-some years ago so I don't know if it counts anymore). On about the same weekend, several people here posted about their first century completions.

If I were a competitive type I'd have felt very bad about our time (it was several hours more that the ride reporters). But, I'm not and, quite frankly, I'm quite pleased with our tortoise-like pace. Our pace was really good for us. We enjoyed the ride and had a great day.

Interestingly, our final time was about what I expected. As we increased our mileage before the ride, I kept track of our average speed on the long rides as well as kept track of how hilly the ride. Using that data I extrapolated what I thought our average speed would be, +/- for weather. We got a good tailwind on the final leg and finished a little faster, but within my guestimation.

So, keep riding, start keeping track and start learning how you respond to various terrains and conditions. Learning to read your conditioning will give you the best estimate of how long it will take you.