24, 12, there's a few 10hr events (run by the excellent and lovely folk of No Fuss), a few 6hr enduros as well and the slight variation on a theme of 100-mile (or 100km) MTB rides, the Merida 100 or Gorrick 100 for example.
24hr race starts at anytime between about midday and 2pm (depending on race), one rider must be on course at any one time, each rider must do a minimum of 2 laps (unless you injure yourself spectacularly), how you arrange the laps is up to you.
Entry forms are usually linked to from here, the threads start on here several weeks beforehand, gives you plenty of time to enter and prepare.
You need good lights and a sense of humour. At times it will be fantastic fun. At other times it will be the darkest hour of your life.