Well, if we're talking about how soon after an event it can happen, the funny thing is that we even have an example that has a negative time there, so to speak. Someone was proclaimed the messiah in heavens BEFORE he actually, you know, got killed and supposedly went to heavens, and all the while he was telling those people that he's not the messiah.
I'm talking of course about David Reubeni. He never was a rabbi or a messiah pretender per se. He was a big time scammer with a lot of chutzpah. He scammed the Pope and the Holy Roman Emperor, and would have gotten away with it too, if it weren't for those pesky kids... err... I mean, if he knew when to stop. He had no interest in religion per se, far as anyone can tell, and to different people he had presented himself as a descentand of Muhammad instead of a representative of Jewish kingdoms.
And I may have mentioned before that his cult took off when a schizophrenic girl dreamed of him being in heavens, and giving her a tour of the heavens, and promising golden thrones in heavens to all those persecuted by the Inquisition. Suddenly a LOT of people wanted to believe that he IS the messiah, reality and details be damned.
Funnily -- if you're into sick gallows humour

-- it should have been clear to anyone who's read the OT that she's a false prophetess, because she failed in what she promised. See, apparently she didn't just see Reubeni, but also Moses and the angels, and was promised that she'll be the one to lead the Jews back into the Holy Land. Instead of that, she and 38 followers were burned alive by the inquisition. False prophet right there. As prophets go, that's an EPIC FAIL right there.
But it didn't matter. Instead of dampening the claim to Reubeni's messiahood, it actually just made even more Jews believe he's the real article.
Where I was wrong before -- since some sources are unclear about the order of events and years and all -- was assuming that this happened after Reubeni's death. I mean, come on... You don't preach that someone is sitting in heavens at the Father's right, as in RIGHT NOW, unless he's dead, right? How stupid would it be to believe someone is in heavens right now although he's in the next village right now, right?
But actually that's exactly what happened. The guy was proclaimed a messiah and was seen IN HEAVENS... while travelling to Portugal to scam the king of Portugal.
To their partial excuse, though, one must remember that his visit did put the Inquisition persecutions on hold. Mainly because the King needed the Jews' money to pay Reubeni, plus it seemed unwise to persecute Jews while trying to ally with some great Jewish empire that supposedly Reubeni represented. So I guess he was a bit of an incidental saviour of some people's mortal asses.
Still, not only he never claimed to be a messiah, he flat out said that he was a military envoy not a messiah, when Dioeo Pires (another guy who had gotten all crazy about Reubeni's being the messiah... although Pires was an uncircumcised gentile

) came to greet him as the messiah. In fact, Reubeni seemed to be by now very circumspect of anyone trying to make him a messiah, since a few people were starting to convert and encouraging apostasy from Xianity... now that could jolly well get him tied to a stake instead of given lots of money. He treated Dioeo Pires very coldly.
Of course, that went exactly like Brian's denying his being the messiah in the Life Of Brian. Pires apparently just got more convinced that Reubeni is the messiah, and thought that his being coldly dismissed was because HE, Pires, was basically not worthy of the messiah's attention. So he went and got circumcised and tried again
To round off the package, Reubeni really was as far from messianic expectations as you can possibly get. He was quite the opposite of expectations of humility, and caring for the poor, and all the other things that other wannabe messiahs made a show of. He travelled in luxury, wore the finest embroidered clothes, and generally knew how to live the good life on other people's money. Now I can see how that may have been good for his persona as a supposed ambassador of a mighty kingdom, but... I'm kinda at a loss about exactly what of that made people think that now THAT is a messiah.
At any rate, there we go: someone as far as you can get from qualifying as a messiah, got proclaimed the messiah, all the while he was around to deny it.
If you thought the Life Of Brian was just taking the piss, it actually happened, folks
