Rave : 2002 : Do Sequels REALLY Suck?
Here's my two cents,
I think what's more important here than the money thrown at these projects is the source material. Allow me to explain...
Jaws (for instance) was written both as a novel and for the screen as a single, self contained and complete story. It's sequels all suffered from various degrees of sucking because they were attempts to add on where no additions were needed. Same for Godfather 3... the story was complete after II, but they felt the need to cash one more check from the franchise.
The sequels you list that don't suck, on the other hand, are good in large measure because they're required to tell the story. Tolkein wrote the Lord of the Rings as a trilogy (plus a prequel) long before anyone thought to make movies out of them. Same for the Harry Potter books. If the Spider-Man sequel is as good as the original it will be because they remain true to the source material.
There are two exceptions that support my rule here... the live action Batman franchise and the prior attempts to make movies from Tolkeins work. In both cases the film makers completely disregarded their sources, in Batman's case by ignoring characterization and in Tolkeins by an attempt to unacceptably compress the story. The results were disastrous. But as we see from the Batman Animated Series and the Peter Jackson films the weakness was not in the basic material, but rather in the people adapting it.
So that's what I think. If your source material is good and you remain true to it there's no real upper limit on how long a franchise can go on. If you're attempting to artificially extend an already complete tale then even one sequel is too many.