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It seems to me, and this isn't written in stone, but it seems to me that it depends on where in their index cycle you were crawled that determines whether you will be folded into the index in a month or two months or more.
Even when Google was on a monthly update, I had sites sit on the sidelines for up to three months before they started ranking. And it seems like Yahoo is following a similar pattern.
The quantity of links seems to have a bearing on how fast I rank, too. Like, if I throw a bunch of links at something it will rank faster than a slow growth of IBLs. Some directories get me favored by MSN but not by Yahoo... funny.