Best answer is, it's complicated.
Hopefully a bit more helpful, if you're quoting a short excerpt of a work, that's considered fair use. Just like with the papers you wrote back in school. As long as you cite the original source, you're ok.
How much is too much? There's no specific number of words, notes, or lines that can be taken without permission. It also depends on what your purpose is. If it's for something in the greater good (e.g., education or scholarship) you have more room to use it than if you do basically the same thing for commercial purposes.
It's safest to get permission first. It's less hassle to act as responsibly as you can and then apologize when you're caught.
