I've noticed with nook books it's highly variable. I've seen samples that are more than a hundred pages, others that are maybe ten.
It's not always the same percentage, though. Wheel of Time books aren't exactly short. One of the ones that had something like a 150 page sample was definitely not significantly longer. If anything it was probably shorter.
ETA: Here's an example. I am using the product details from Barnes and Noble's website for consistency.
The Paladin of Souls by Lois McMaster Bujold - Total # of pages: 496. # of sample pages: 128
The Eye of the World (Wheel of Time book 1) by Robert Jordan - Total # of pages: 832. # of sample pages: 4 (!? - that doesn't even get you past the ridiculous list of blurbs praising the series!)
I'm not the only one! I've noticed this too, and it makes me so angry! I mean, why offer a free sample if you don't even get to read a sample of the actual story? I'd rather there be no offer of a sample than be misled into thinking I'm actually going to get to the first pages of the story.
I got so irritated with a sample that was ten pages of acknowledgements and copyright infringement warnings that I deleted it from my phone.The last page was the chapter number.
It seems likely that publishers are also running into this issue, but if no one actually checks it on a nook they won't notice the discrepancy, as it comes up properly via the web viewer. So that might explain why The Eye of the World only has a 4 page sample, which includes none of the actual text of the book. Hmm.