How long can a bad trip on weed last?

Varies...: Most bad trips last hours. But some "bad trips" can last years as weed increases risk of developing a psychosis like schizophrenia. If your bad trip involves depression you may feel suicidal in which case the bad trip could be very short lived.

Depends: Marijuana's effects usually last from one to three hours. However, in some cases there can be psychiatric effects that last for years. Because it can trigger onset of some psychiatric conditions - longer lasting symptoms should be medically evaluated.