For me, it varies. Sometimes I feel like writing something that is entirely subjective, sometimes I don't.
The issue of in-game GM revision doesn't really apply, because either way, I tend to have areas that are deliberately left unexplained and undetailed for just that kind of thing. That way the stuff that I've labored the most on and consider the most important stays more or less mine, and the things that I want to be surprised about later are clearly marked and left completely open for the GM, who will hopefully use 'em to surprise me.
I'll confess, though, that I don't really bother with elaborate backgrounds for D&D characters. For other settings, I'm usually willing to spend a lot of time putting together a background with lots of interesting hooks and details and funny stories. But the most I'll do for D&D is two or three paragraph's worth, largely devoted to answering the two big questions--where did the PC come from and why are they adventuring?--and that's all. The rest is vague or made up on the spot if the GM asks later. There are a few possible reasons why I do this.
It might be because we typically start new games at 1st level, and I figure that anything that happens between level 1 and level 10 is bound to be about a trillion times more interesting than what happened before level 1, so why waste time on writing about the boring part of the character's life?
And on top of that, I have an expectation of PC mortality in D&D that I don't have in other systems; it's so damn easy to end up with a dead character with no means of raising them that I don't want to invest more than an hour in working up a background for them. And really, half an hour is a more reasonable amount anyway.
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also, i don't really identify with or even like fantasy settings that much
ryan