I'm sure it depends on the amount of cotton and even that will probably vary by brand. See if you can find out the brand and look it up online. The Hobb's 80/20 notes 3-5% shrinkage. I do believe I've seen other brands that were 70/30 cotton to poly or 50/50. The more cotton in the content, the more shrinkage probably.
That said I LOVE the Hobb's 80/20. I used to only use Warm n Natural, but now I usually use the Hobb's. It handles similar, but drapes better. I find it more comfortable to snuggle up under. The Warm n Natural has a little stiffer feel to it.