When will the next mass extinction occur?

This is answers.com, not the psychic network.Nobody knows. Various people have predicted various things, but until one of them happens, we won't know for sure.

At one time it was believed that mass extinctions were periodic, and some analyses suggested we were about due for one ... but as more data has come in, it now looks like the periodicity may have been a statistical coincidence, and that smaller extinction events occur all the time. There was a relatively recent one in the oceans about two million years ago, for example (the last really major one was the K-Pg event 66 million years ago, widely thought to have been caused by an asteroid impact).