I think the time-frame depends on who is involved in the title picture.
For instance, as has already been said, Randy Orton and John Cena can hold the title for long periods of time and never seem wrong. The reason being is that they are both built up to be relatively unstoppable in the ring. This can also go the other way in that any other wrestler holding one of the World Titles can have a 2-day reign if John Cena or Randy Orton is their immediate challenger. Others have been "unstoppable", but never in the way that the top guy gets to be. The best way to put it, I think, is this. The top face is always the "irresistible force" and the big heel of the hour is the "immovable object". Unstoppable in their own ways, but when they clash, Hogan always seems to come out looking bigger and better.
They are different from guys like Kane and Big Show, in that neither Kane nor Big Show has been built as "unstoppable" in years. Kane was, in his debut and for quite some time after, very believable as someone who could have been unstoppable and a great heel champion, but they pulled a Christian there and we never really got to see how good a champion he could be until this past year. He was good enough last year when he was past his prime so he definitely had potential to be amazing in his earlier days. But, because it was Austin who beat him (who as the top guy of the top also had to be unstoppable in his own right) it wasn't unbelievable that Kane lost the title. At the same time, though, we've seen Kane lose to much smaller guys, clean, many times during the Attitude era and more recently. The only reason Kane still exists after losses to little guys is his staying power. He was the first character of his kind and has stuck in our minds ever since. Same goes for Big Show.
I honestly can't remember the last time Big Show was built as truly unstoppable. I think it has something to do with his more current persona tainting his debut, when he seemed much more menacing than he is. I will say though that his most recent top title reigns were ended by Brock Lesnar and Bobby Lashley, respectively. Brock was a much more unstoppable force than Big Show ever was, and Lashley was destined to be Brock 2.0, I think. You might also say Undertaker &/or Edge in certain instances.
I personally believe that at this point in time Big Zeke or Mason Ryan could go either way. Both are dominant forces and have never been portrayed as anything otherwise. However, all it takes is one clean loss to Rey Mysterio to Mordecai you out of existence. I believe there is still a far bit of disbelief in most longtime wrestling fans that a smart wrestler like Mysterio (which is what we've been taught that anybody, regardless of their name or whether the are face/heel, has to be to win the top belts, usually) can't find a way to topple a bigger wrestler believably moreso than a new-age Hogan.
I could see Rey Mysterio cleanly beating Mason Ryan with the title on the line before I could see him beating John Cena, it's just how I've been conditioned as a fan since 2005. In classic terms, I could see Bret destroy Yokozuna before I would ever see him go clean over Hogan. This could also have a lot to do with the heel/face factor. Mysterio, as a face, seems more likely to triumph over Ryan, as a heel, in a David/Goliath sense than he does to defeat another (arguably bigger face) in John Cena.
Back on track, I think Orton and Cena, as faces of the brands, are currently alone in being people who can both have a lengthy title reign and end someone else's reign rather quickly as well. In the days of old, I think people who were legitimately believable as being able to do the same would be Hogan, Austin, Rock, Triple H, Lesnar, Batista and, had he stayed around, eventually Bobby Lashley. They were all just as believable as year-long champions or ruiners of title reigns for others at just about any point in their main event careers. I would like for newer stars to step into this category, but until they actually do, it's insanely hard to say who it could be. Look back to 2003 when Orton and Cena were new - nobody could have guessed that they would go where they did. That's why it's hard to say whether it would be Miz, Morrison, Ziggler, Rhodes, etc. who will eventually be that Hogan-esque guy.
Then there's the other side of things. If you're going to end a title reign extremely prematurely then you have 4 options - Orton, Cena, a top/future top heel cheating in a big way or a multiple man match where the champion is ganged up on. Nobody is going to believe that a main event superstar (regardless of who they are, at this point) is going to lose their recently-won title in a relatively clean match unless it's to Orton or Cena. They just always seem to come through when it comes to size-disadvantage.
Furthermore, when it comes to guys like Dolph Ziggler, Christian and those similar, I find that a month-
long reign is not a detrimental thing in any way. As long as it is played correctly, it can be very beneficial to any character to hold the top title as a transitional champion the first time, just to see if they can garner the support of the fans (without a huge controversy, which I believe is the only reason everybody is saying they wanted Christian to hold the title forever - he was good enough for a month or two, but after that I would have been fine with him not holding the title to give someone else a rub).
This is why I wish Ziggler had won the title at Royal Rumble with interference from Vickie or not at all. His 11 minutes didn't do anything for anybody, and Edge needed neither the extra reign nor a win to increase his believability. He could have easily lost it the next month at Elimination Chamber to Edge. It would have culminated in Edge getting the extra reign anyway and could have possibly made it seem to fans that if he could lose to Ziggler in January, he could lose to Del Rio as well. I also wish Christian had been placed in a feud with someone like CM Punk (who should have been drafted, to keep top heels in balance on SmackDown!) who he could have dropped the title to at the end of a month or two. After that point, Punk/Orton could have gone right to Summerslam and would have been great, I think.
All that said, I prefer when a champion has 3-5 months to flesh out a title reign and tend to hope that (unless they've done a spectacular job as champion - which I believe Miz was closest to recently) by the 6th month, we are at a Big 4 PPV where there can be a big title switch (or any event, really, but WM, SummerSlam, Survivor Series & The Rumble always seem like a better place to switch a top title, just IMO - no real reason behind it.) My favourite example of why this is the perfect time-frame is CM Punk's post-Jeff Hardy World Title reign. He was at the top of his game and was the greatest thing in wrestling for that short period where he was able to gloat about having removed Jeff Hardy (someone every kid loved) from the company. He did everything he needed to do to increase his credibility before he lost it to the Undertaker and never looked out of place doing it.
