JSON is minimal in comparison to XML. You could easily cover it in an hour (perhaps two). Although XML is bigger than JSON, you should be able to cover the language features in a couple of hours. It may take a bit longer to cover validation in terms of Document Type Definitions and schemas. If you're interested in covering parsing, XML transformations, the Document Object Model, and so forth, expect the course to run over a few more days.
When discussing XML basics, I think that the trickiest parts for the newbie are namespaces with namespace prefixes, CDATA sections, and validation.