According to GAMEFAQ, the FFX and FFX2 Ultimania guides post only 5 "Calm" periods (below), with High Summoners appearing in intervals of 600 years thru 90 years. Now, here is my problem with it:

- We see that Yu Yevon takes only 9 years to rebuild Sin, as it was all it took for Sin to reappear after Braska defeated it.

- Does this means Sin is "alive" and loose around Spira for centuries ?

IMHO that is impossible. If Sin kills as we see it doing in FFX every single year during those large periods, the planet's population would be extinct after a while, say, 200 years

- Did Yu Yevon take much longer to rebuild Sin initially ? i.e. did he take, say, 500 years to create the second Sin ?

IMHO that is impossible. A threat that has not reappeared for a generation or two would not command such fear and devotion from people. Bevelle and Yevon's Church would lose a lot of it's prestige and influence after 100 - 150 years without Sin. The more "costly" tenets of the Church would be abandoned, i.e, Summoner training would be abandoned and so would be the machina prohibition

As proof I submit the Catholic Church. Sure we still have people who believe Jesus will come back, even after 2,000 years of wait, but the power of the Church to run our daily life has been gone for a long time. There are very few people who still refrain from eating meat on Fridays and even less people who abide by all the other tenets in the Bible.

There is no way people in Spira would still refrain from using machina after 200 years of no Sin!!!

So what gives ?

Approximately 1000 years ago

- Using the power of these fayth, Yevon summons Dream Zanarkand - the memory of Zanarkand. He himself becomes Yu Yevon, and creates the first Sin. (The birth of Sin.)

- Yunalesca, a summoner from Zanarkand, turns her husband Zaon into a fayth and destroys Sin with the Final Summoning. She remains on Spira as an Unsent

- The second Sin appears publicly with Yunalesca's Final Aeon (Zaon) as its core. It destroys the remaining machina civilisations.

600 years pass... With Sin killing thousands every year ?

Approximately 400 years ago

- Summoner Gandof defeats Sin with his Final Summoning and becomes the first high summoner in history.

(This gives credence to Yevon's teachings, and doubts about the Temples subside.)

GANDOF'S CALM

- The third Sin appears publicly, with Gandof's Final Aeon as its core.

170 years of Sin Killing ?!?!?

Approximately 230 years ago

- Summoner Ohalland, a former blitzball player, defeats Sin with his Final Summoning and becomes a high summoner.

OHALLAND'S CALM

- The fourth Sin appears publicly, with Ohalland's Final Aeon as its core.

130 years of Sin Killing ?!?!?

Approximately 100 years ago

- Summoner Yocun, a former Crusader, defeats Sin with her Final Summoning and becomes a high summoner.

YOCUN'S CALM

- The fifth Sin appears publicly, with Yocun's Final Aeon as its core.

90 years of Sin Killing ?!?!?

10 years ago

- Summoner Braska defeats Sin using Jecht as the fayth for his Final Summoning, and becomes a high summoner.

BRASKA'S CALM

9 years ago

- The sixth Sin appears publicly, with Braska's Final Aeon (Jecht) as its core.

1 year ago

Sin reappears and kills Chappu

http://forums.eyesonff.com/showthread.php?p=1496935