As a DM: I expect zero backstory beyond what your character generation rolls and-or choices (age, secondary skill/past profession, race/culture, class, languages spoken, etc.) might provide. If someone wants to do up an elaborate backstory that's fine, but there's still every possibility the character's going to die two sessions in; so my usual advice is don't waste yer time until the character has survived long enough to make such effort worthwhile. I'm certainly not going to listen to complaints about said wasted effort.
As a player: I'll use the info given from the choices/rolls above to put together a possible backstory* in my own mind, which - if it ever becomes relevant - I'll reveal in character as the game goes along. I do it this way to leave it a bit malleable to incorporate things I learn about the game world along the way.
* - anything that would give me undue advantage or knowledge (as either player or character) obviously either gets cleared with the DM or doesn't happen.
An example: I've an active character - a magic user - who is from a Roman-like society. Didn't bother with much of a background for ages, but on learning more about how this Roman-like society worked I decided she had done a tour or two with the Legions as a staff mage and (what amounts to) officer cadet, and with the DM's permission invented some of her ex-commanders and some backstory involving them...and why she left the Legions. The other players/characters know very few specifics and not many generalities, yet this backstory largely makes her what she is and drives how she relates to others in the party.
Lan-"and sometimes I don't even bother doing this much, but instead just let the character's backstory define itself during play"-efan