Where's your line?

Where do you draw the line when you're late for class and must decide whether to walk in late or just miss it altogether? The latest I've ever walked into a class was 15 minutes late, but it was a day where my hour commute turned into a 2 1/2 hour commute because of snow and ice. So I felt justified. But I don't think I'd come into a class more than 10 minutes late unless I felt that others were going to be late as well. It just seems disruptive to me.

I bring up this question because a classmate is consistently 20-25 minutes late for an hour and forty minute class. It happens with such frequency that I'm beginning to think that this classmate has something else scheduled during class. Which begs the question, why not just take classes when you have free time? [But then again, maybe there is a valid excuse and I'm being judgmental.]

Actually, what sparked this thought was a girl who walked into my 2 hour class at the hour and forty minute mark. She came into the room, set her stuff down, and then walked out. At first I thought, why come into class with only 20 minutes left if you're just going leave? When she never returned, I realized that she wasn't in fact late to class, but felt that she needed to come set her stuff down before her next class. While our class was still going. Either she didn't notice, or she didn't care.