Spare time to attend lectures for Grad students?

This question is addressed to (past or present) graduate students doing MSc or Phd. I am aware that there isn't any coursework for these students? Or maybe very rare amount? In my uni there isn't any for maths or physics grad students.

Do you spend most of your time doing your research? Do you find time to attend (although not neccessarily enroll since they don't check enrollments during lectures) some of the undergrad lectures that you did not do when you were undergrads? If so how many of those subjects do you think you can fit in per semester? I am assuming you are doing all the coursework in those subjects and not only listen.

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