I think this question should be no one knows the answer. The last round of the bear market was ended at the beginning of 2017 and originated at the end of 2013, it lasted for about three years. But this round of bear market is only eight months until now, its very likely that it will last longer.
there's a difference between a bear market and a flat or bored market. the bear market ended in 2015. after that it was a sideways/dead market until it started to move up again.
i expect it to last into next year. when every single sign of life is still jumped on as a sign of the reversal of everything that tell us confidence is still too high.