Once one has mastered most of a course, they might profitably skim through their notes at the end to summarize outcomes they saw and find most useful and interesting. Those things along with the summaries of their Cornell notes might then be useful zettels to keep in the long run. A zettelkasten practice like that of Niklas Luhmann is more useful when one already has a strong lay of the land and they’re attempting to do the work of expanding on the boundaries of new areas of knowledge.
If you are a student contemplating creating a zettelkasten, then the bulk of your notes probably ought to be short snippets kept with your bibliography notes and should not be individual zettels. By this I mean specifically that you might have a bibliographic note (reference note or literature note) for each individual lecture with some fleeting notes about it kept with that card. Then if necessary, you’ll probably only have one or two zettels or permanent notes out of each lecture. If you’re attempting to create 30 permanent notes a day and interlink them all, then you’re going to find yourself overworked and overwhelmed within just a few days.