for me, on medium history on default worldgen its 250. or short history.... i forget =\
The default history length on the default settings is 250 years. I don't know what label that has.
The default history length in the "advanced" settings is 1050 years, which is also the former default in the default settings. It is my opinion that Toady simply neglected to change this one, when he changed the other one to 250 years.
1050 isn't so much the "default length" as it is the "cut-off length." Worldgen is supposed to stop after a certain number of megabeasts are dead, in the Age of Heroes- in 40d this regularly happened about the 250 year mark, give or take half a century depending on the vagaries of your world. With the material rewrite (along with the other host of changes in 2010,) however, megabeasts stopped dying in any appreciable number during world generation, so the world would run to the hard stop date- 1050.
Presumably, this isn't enough of an issue for Toady to devote much time to fixing yet, but when he did the more detailed "simple" Create-a-World interface he made the default 250, the way it was in 40d. Long enough for some interesting things to happen without being so long that races start going extinct.
In short, the default advanced parameters are a holdover from previous versions (since you're already going to muck with them, why change the default?) while the regular Create New World interface is a recent addition (about the time he added the ability to adjust mineral scarcity.)