How long do red eyes last when a teen is high of weed?

Several Hours. Usually 2-4 hours from the time of last use, though that can vary considerably depending on the amount used, the potency, etc. Also, cannabis's ocular effects will vary from person to person.
Not long. Probably not more than a few hours or so. Not everyone develops red eyes from cannabis use. I would not guage a person's use on whether his or her eyes were red. If there is a concern, other messures such as urine drug screening are much more accurate. You need to ask what you will do if a test is positive before doing such a test. Good luck.