I'm watching Fate/Zero, and it talks about how the Holy Grail Wars are supposed to be every 60 years. However, it's set in the 1990s, and the Fifth Holy Grail War takes place in the 2000s. Why were there only 10 years(ish) in between instead of 60? Is this explained in Fate/Stay Night or any of the other Fate stories?
As i explained in this answer (Under Kiritsugu's Plan), it was because there was a build up of Prana in the Greater Grail.
Normal Procedure would be that by the end of a War the Prana (particularly Mana) in the Greater Grail would have been spent and thus take 60 years to naturally replenish. Under Fuyuki - Procedure
The Great Grail requires sixty years in order to amass enough mana to summon Servants, making the planning period span over generations. If the Grail is unable to utilize its energy, that which is leftover can reduce the time to as little as a decade, such as in the case of the period between the Fourth and Fifth Wars.
As for where it was mentioned, the citation reference of the section on the Wikia Pages points to Fate/complete material III: World material - FAQ with Nasu
Q: Even though it should only happen every 60 years, was the 10 year gap between the Fourth and Fifth Holy Grail Wars as short as it was because the Lesser Grail was destroyed before the Greater Grail could direct the accumulated mana to establish its pathway trapped some unused mana in the Grail?
A: That’s true. In fact, you could say that the function of the Greater Grail was already muddled when when it was impregnated with Avenger in the Third War.
So from this there are 2 causes for the Mana not having been spent.
- Lesser Grail was destroyed before the Greater Grail could direct the accumulated mana
- Avenger, possibly self-aware, perverting the system by withholding Mana as there was still a 60 year gap between the 3rd and 4th Wars (as Avenger was killed early in the 3rd war)