First a little math exercise that I have done since I was an undergrad. Let's take a student who take five courses per quarter. There are about 28 hours/quarter and 3 quarters. Northwestern University tuition is $35,064, coming to $83.49/hour. If you have 30 students that amounts to a bit over $25K of tuition spent on each hour you teach.
This is false math, but students are often paying the big bucks for college, including the opportunity to be taught by leading researchers in the field. You shouldn't deprive them on a regular basis.
Nevertheless sometimes you have a workshop or a conference that you don't want to miss. As a general rule you shouldn't skip enough classes that students feel cheated. There are many factors that should be taken into consideration.
- Why are you missing the class? Do you have a family emergency? Are you a speaker at some conference? Are you visiting another university? Doing consulting work? Taking a vacation?
- What level of class? Skipping an undergraduate class is different than skipping a graduate seminar.
- Is there an easy make-up time that works well for the students? Can you find a replacement instructor?
- What is the standard at your university? Could be different between schools and departments as well. My business school friends cannot miss classes. MBAs get upset easily.
- What is your current status? Missing classes is a sign of irresponsibility. It can hurt your tenure case if you do it too much.