As someone on this forum once said, "It's not the info, it's how it's dumped." I would add "and where." Of course how much information is too much or just enough will depend on the individual reader. How much do they want to know about the background? How much worldbuilding do they want? From all the related discussions we've had, I think it's clear that we can never please everybody; the important thing is to do it as gracefully as possible, so that readers who want the information won't (as HareBrain says) be thrown out of the story.
Sorry, Hex, but it
is complicated, and there are no hard and fast rules. You have to develop your instincts for what will work for the kind of book you are writing and the kind of readers you are trying to reach -- which ought to be readers just like you. However, I've met people who say they don't like description at all, but when asked to name their favorite authors they'll come up with a list of writers who use a lot of description. I think those writers slip it past them, so that they think they are imagining everything for themselves. So look carefully at your favorite writers, and see what they do and how they do it. They may use more background information than you think; or their writing may be just as sparse as you think it is.
You are right that an info-dump can be as short as a sentence or two. It depends on whether it is intrusive or not. But for readers who are hungry for the background information (and if it is well-written) several pages may not be too much.
I've met a lot of aspiring writers who labor under the delusion that if they put it into dialogue it isn't info-dump. Peter, I love your example of a dialogue info-dump. And MemoryTale, I love your example of how it can be made to work.
And then there is the notorious, "As you know, Dr. Watson ..." where characters tell each other things that they already know, which is almost always clunky and obvious.
But this, too, can work if the characters are arguing.
"If we start now we'll make the bridge."
"But it's twenty miles away, and the enemy has a head start. What will we do if they get there first?"
"We have better horses. They'll never outrace us."
"It's still a risk. What if we go south to the ford instead?"
"The rains have been heavy this winter. It may be impossible to cross."
Well, perhaps not a brilliant example, but you get the idea. They're each trying to make their point by bringing up information that the other one knows perfectly well.