On The Calm and The Final Summoning

So I remember during my last playthrough on the HD remaster a couple of years back, wondering how long the Calm lasts. Is this ever clarified? I know there have been 4 high summoners prior to Yuna, but there's also been 1000 years of Sin. So that's one summoner every 250 years.
At first you might think that this is because the calm lasts for a couple of hundred years, but since Braska was the last one and Sin is already back, that throws that theory out of the window. So for just under 1000 years there were only 3 calms? If they last around 10-20 years then why weren't there any other High Summoners stopping Sin? Why aren't there a few dozen?

This brings me to my next question. I seem to remember them hinting in the game that the final summoning doesn't necessarily mean that Sin will be defeated. That it's just the only chance to defeat Sin in battle, but doesn't always work. Is this ever confirmed? It would help explain why there have been very few Calms and High Summoners. It would also explain all the smaller statues in the temples. Perhaps they're just those who went up against Sin and failed. I don't know. I'm looking for answers from someone who's more well versed on the world of FFX.

I think there were more High Summoners than the series outright states. But this is something that's bothered me for a long time! Because you never hear anything about them, and some of ones we hear about lived a couple of hundred years ago. But that really makes no sense at all, if there have only been four. Maybe the tiny statues in the temples are past ones, I dunno. Haha, also just reread your post and saw you mention the tiny statues.

As for your second question, yes, the game confirms it. I believe it's hinted O'aka and Wantz' sister died during the Final Summoning (although she might have died on the road, like Lulu's first summoner). But the second? time you meet Belgemine, Yuna comments she's so powerful and will surely beat Sin, and Belgemine tells her that she tried but failed. At this point we don't know all that Final Summoning requires, nor does Yuna--she only knows that the Final Aeon will kill her. However, after you beat Belgemine the last time in Remiem Temple, she reveals that she's also an unsent, and having guided you and helped you grow, she feels her failure wasn't a waste. So yes, we do get confirmation that not all summoners who call the Final Aeon necessarily defeat Sin.

Thanks for the clarification on the second question.
If there have been other high summoners, why aren't they as worshipped as the main four? Braska makes sense, since he was the last one, but why are the other three given pride of place?

Yet another question bugging me too. When the calm rolls round, why don't summoners start their journey then? That way, when the calm ends, Sin has only just returned and a summoner can hit it straight away, with it doing minimal damage to Spira.
My only guess is that it's something to do with the fayth not being active while Yu Yevon is rebuilding Sin, but I don't think that's anything more than a guess on my part.

Wasn't there some mention about people being in fear of when Sin returns during a Calm? However it was worded it seemed to imply that not all Calms are the same length.

I remember there being something in the game saying that each Calm is shorter than the last.

I always assumed different Sins had slightly different temperaments depending on who "became" Sin.

i dont really know, it was just an assumption. i assumed jecht-sin was keen to come back and hang with his lazy son.

or its just a big old plot hole we can fill in with head-canon like usual