With the caveat that my knowledge of Ukraine is rather weak (I've only been to Kiev once), my best guess it that they won't be able to conquer all of it - Ukraine is huge, so eventually I think it would be split it into two parts.

Within Ukraine there's a decent divide between those who are pro Russian in the south and those who would like to retain the Ukrainian language and culture, mostly in the north, so it wouldn't surprise me if Russia manages to retain the southern part, but ends up having trouble with North-Western Ukraine.

That said, this isn't a game of Civ and I hope the Ukrainian people won't have to go through that horror.