For organized road cycling events, there are a series of typical distances that travel to complete them. Since cycling is generally considered an endurance event, a variety of distance choices are available for the rider, from beginner to advanced rider.
Typical ride distances (all approximate) are as follows:
8 mi/13 km - for fun and beginner rides
12 mi/19 km - longer beginner ride
20-25 mi/32-40 km - first "serious" distance ride distance; frequently used as a short ride to train for longer rides
35 mi/56 km - moderate distance ride
45 mi/72 km - longer distance ride
62.5 mi/100 km - the metric century Most people, with appropriate training, can ride such. Achieving such is nevertheless a personal milestone.
70 mi/112 km - slightly longer than a metric, for a bit of additional challenge
100 mi/160 km - the full century; quite a challenge, considered the longest typical 1-day endurance event
130 mi/200 km - the double metric
190 mi/300 km - the triple metric
200 mi/320 km - the double century
As one can see, the range of rides varies tremendously. There are also multi-day events such as the BRAT (Bike Ride Across Tennessee) from September 11-17 of this year. Longer supported rides of 40 mi or more typically have one or more rest stops along the way for cyclists to obtain additional water and food, as well as taking a break.
How long does it take to ride such rides? Cyclist speeds vary tremendously, and the amount of energy needed to travel a distance is quite sensitive to speed. A typical slow speed is around 10 mi/hr, and 18-20 mi/hr is typical for elite cyclists. For group rides, frequently people will specify whether the ride is a "no-drop" ride, where no slow riders will be left behind, or not.