An ultramarathon is any race that is longer than the traditional marathon, which is 42.195 kilometers. There are generally two types of ultramarathons. One is defined by distance whilst others are defined by a period.
Time-based events can range from anything between 6 hours and 10 days. There is no fixed amount of time. If the marathon is based on time, then the specified number of hours is determined by the race.
Timed events are normally run on a short track or a road course. Time spent is prescribed and differs from one race to another.