I would say, it is really dependent on the character, him/herself (and to some extent as others have noted, the writing ability of the DM). With good writing, I'll read an entire sourcebook. But realistically, for just getting started, it really should be accomplished in anywhere from 2-5 pages and cover the following (general) bases:
1) Where I come from. I should have a fairly thorough knowledge of the town, terrain, landmarks, creatures, notable NPCs and other peoples of my home. How far out that knowledge extends depends on my background. Bare minimum, I probably know little beyond a day's travel from my village, the closest town with a market and probably the closest major and/or capital city of my realm. The name of my realm/region/barony/duchy and greater kingdom (if there is one.) and it can be safely assumed that I know who is the ruler of my land (be it local noble, king, high priest, etc...all rulers make sure their people know who they are.)
If I'm a mercenary (or perhaps a bard) who's traveled a certain part of the world, then I may know more countries/cultures/peoples/further afield. If I'm a wizard or cleric, I probably have knowledge of the names and rulers of most (if not all) civilized countries/nations/peoples. Most of this info can/should be given in a single line or two.
2) Races. What race am I and how much info is there available on my culture? What races are known (by my/most people) to be in the world as a whole? In my local region/realm? Have I ever
seen an elf before? A gnome? Ever heard of a tiefling or dragonborn (if you use such abominations

? Naturally, if I am a member of a race other than human than I have a different knowledge/perspective on this.
Also, naturally, if I play an elf (or some-other long lived race) I'm going to possess extensive background/history information...since I've been around for 400 (or 1,000) years or so...Whether this is doled out at the beginning or inserted in-game as needed/"it comes up" is entirely up to you and how much you, as the DM, work out beforehand. But do not forget,
in play, what races you have at the table...a dwarven education/experience is not the same thing as that of the elven mage from the south country or the human barbarian from the east.
3) I have some knowledge of my realm's religion...even if my character him/herself is an agnostic or atheist or just not a particularly religious person. I will know, at least, the god (few gods) worshiped in my local area/needed in the every day. I will know a national deity (i.e. the acceptable religion of the state), if one exists. I, undoubtedly, have been taught/warned/heard legends growing up of some "dark force or god" that is the root of all evil that I must avoid (or appease) at all costs. If there are large or long established holy orders around (i.e. the Knights of Thatland or the Holy Sisters of Overthehill Abbey), I have probably heard about them.
Obviously, if I'm a cleric (or have the "Knowledge: Religion" skill) then I would have a fuller understanding of the entire pantheon, and those of other realms/nations (if applicable), possibly/probably lost or dead deities who are no longer worshiped (though this might also be gleaned from "Knowledge: History, Ancient"), heard or read whispers of major evils (demon princes and archdevils known to assault the material world).
4) Class background. Does my class have any specific traits, culture, skills, knowledge that other PCs wouldn't have (beyond what the PHB says, I mean)?
I.e. Is my wizard part of a Guild? Are there different guilds for different types of mages/regions/countries? How do they interact? Are all paladins a member of a particular order? How affluent/influential is the temple of my cleric's deity? Do all temples of God A accept/host clerics of God A as members of the same order? How does my temple/followers of my god view and interact with followers of other deities? Did my rogue (prior to her "life of respectability" as an adventurer) belong to a band of outlaws? Did my bard go to college?
What knowledge, services or products does my character have (or have access to) as a result of this affiliation (if any)? How large is the organization? Where are their headquarters/towers/hideouts? Do I know (and how well) any higher-ups in the organization? Do we get along?
5) Worldwide Organizations & Persons of Note (if any). Are there any truly "famous" groups, creatures or persons whose names live in bardic song, ancient tomes, campfire stories or terrifying infamy spanning history and/or the realms?
Is there a massive Assassins guild across the realm that makes the blood run cold at the very mention of their name? Are the Paladins of Thatland known (to my character) as great heroes or ravaging invaders? A tyrannical warlord who almost conquered the whole world a 1,000 years ago? A Merchants guild that controls economy over a large region? The ancient wyrm, Realbignscaley, that holds his massive hoard in a lair beneath the Highcloudy Mountains..."C'mon, you've heard of Realbignscaley, ev'rybody back home has."
6) Currency. For some people this is a non-issue, others use it a lot. It's up to how you run your game. What is the currency which I have/use? Is this universal/throughout the world? When I go from Kingdom A to Trade City B will they take my money/coinage? If they differ, do I know what the different currencies are, what they're worth and how to interchange them?
All in all...give me my "common knowledge"...and that should be able to be accomplished, more or less, in 2-5 pages. As noted re: "long-lived races", if it requires 10 pages of world history and legend, alone...so be it. If the DMs a good writer with a cool setting, I'll read all day. But most of the basic bases should/could/can be covered within 5 pages.
Have fun and happy wordsmithing.

--Steel Dragons