Boy...that is a hard question to answer. They range from less than 5% to upwards of 30%...maybe. I would hope that this is close to 20% of the Skyrim content...I hope. Again it is anybody's guess.
EDIt : The more I think about it...If we use projected hrs of gameplay as a medium. Skyrim is around 300. Shivering Isles was around 40-45. They said that, if I understand, the Skyrim DLCs would be compared to but less than SI. If we assume that it will be around 30-36 hrs, then that is 10-12%.