In order to calculate your time for a century you need to take into consideration the actual length (some centuries are longer than 100 miles) and the elevation gain. If the route is hilly, how fast do you normally climb hills? 10 mph, 8, 5? Considering the descent and give yourself 10 - 15 for a sag stop every 20 miles. Fast riders can do a century easily in 5 - 5 1/2 hours. Average time is around 6 1/2 - 7 and for hilly or first time centuries expect 8 - 10 hours. My fastest century (I'm a slow climber) was just under 7 hours (that includes all the stops) and the slowest was 10 1/2 hours when I did The Ride Around The Bear which had 9000' of climbing. I had quite a few stops on that one. Also I don't like to stop more than 5 maybe 10 minutes for a sag. Some folks like to take longer. So if you normally do a 50 mile hilly club ride in 5 hours, expect century times closer to 10 hours. If the route is flatter, and you normally do flat 50 milers in say 3 1/2 - 4 hours expect a 7 - 8 hours ride.