When is it appropriate to walk into class late? The answer depends on your schedule and the class you're taking. If you arrive 10 minutes late, you'll probably miss class. If you're more than half an hour late, you might want to reconsider. If you're more than 10 minutes late, you may want to consider requesting a late pass.
Is it too late to walk into a class?
The best thing to do if you're running late is to email your professor in advance of class. If the class is over 40 students, walking in without letting the professor know is highly disruptive. You should also avoid slamming the door. Instead, try to get into the lesson as quickly as you can. If you're still unsure, try to listen for a few minutes to get a sense of the material.
If you're walking in late, you're going to miss some important information in the first few minutes of class. The professor will typically make announcements before lectures. It's also likely that you'll miss reading assignments or requirements. If possible, try to contact your classmates to find out what you missed.