[TwilightsCall]

The grail war happens every sixty years, because thats the amount of time necessary for the holy grail to gather enough prana to support one. The 5th Grail War was different, because

Spoiler for Fate Zero:

at the end of the fourth war, no one actually won, so the magical energy the Grail had stored never ended up being used. The Fifth war started after a 10 year interval instead of a 60 year interval because it took relatively little time for it to gather the required amount of magical energy.