CSS is everywhere. Web development nowadays couldn’t be conceived without CSS.
I have taught HTML + CSS the first time back in 1999 – so I’ve probably spent quite some time with the language.
You can study CSS in 1 day or 1 month – depending on the level of depth you need. This year I have taught CSS over 2 weeks in Hangzhou Dianzi University in China and I have used John Ducket book, “HTML & CSS: Design and Build Web Sites” and slides as a starting point.
On top of his material, I have prepared a few hundred slides with both theory and activities.
The slides are available here – feel free to use, modify and redistribute under the terms of the Creative Commons Attribution-ShareAlike 3.0 Unported License.