One question I'm really anxious to have answered is this: if Wizards heard about your world (or whatever other publisher suits your fancy) and decided they wanted to publish it, would you be able to package the whole thing into one of their double column 320 page campaign guides?
No.
But on the other hand, I've never formalized things as much as I'd want. There's lots of stuff I know and stuff I invent on the fly based on my general feel for the setting, but I've never tried to write up a campaign guide per se. And much of my high school and college notes on the far flung regions of the world (the interior of the big continent of Sartha, and almost all of its west coast, for example) that I never actually gamed in but only imagined is lost and my memories of those places are rather hazy. If I tried to write a world guide, I'd have to reinvent so much both because I've forgotten so many details and because probably if I did have the notes my sensibilities from that of a 16 year old DM would have changed so much (not that I don't admire the creativity of my 16 year old self, who was in some ways more daring than I am now).
My feeling has always been that it would take about 3-4 such books: one on geography, one on history, a third on religion, and the last to cover unique spells, classes, rules, and other mechanical ecetera.
I'm sure there is no set amount of pages needed to properly convey a world, but when I first started making this a few weeks ago, my goal was roughly 200 pages of information, including room to post pics, and package it all as a PDF. Now however, as I do more work, really think about what I want to do, I expect I could need far more than that.
Once you do start to formalize things, it's always bigger than you think it is. For example, the above estimate is a broad outline that would give only a paragraph of text on most regions of the world, and maybe real detail on only 1 or 2. To really fill things out would probably require another 50-100 100 page supplements detailing the culture and history the of various nations and regions of the world, and what is actually to be found there of interest to an adventurer (or a DM). And even that is a broad overview, the real detail would be provided by the DM's that adopt your setting.
So what say you, would 300-500 pages of material sound about right to you, or does this seem more like the high hopes of someone who will likely get burnt out around 50 pages because they've been too ambitious?
I would say that from a pragmatic standpoint, you are starting in the wrong place. Instead of creating a world guide, I'd start by creating a village or town guide. You only need as much big picture as informs your specific setting. After that, start pouring your details into the place you want things to happen - a rural setting or a small city state is a good place to start. Then grow that area as needed as the PC's grow and explore. It would take devoting your whole life to world building to really complete a world guide, especially if it wasn't your job and you weren't getting paid to do it 40+ hours a week.