You could start by reading something like "An Overview of Bull and Bear Markets" to get some understanding of what bull run / bull market and the opposite (bearish) means.

And as an answer for you: it's hard/next to impossible to predict clearly when will it end. Right now the price is on up trend. Corrections happen now and then. At some point the down trend will be bigger and longer than a correction and we will actually leave the bull run and go into bearish market.

Take a look to a bitcoin chart (set it for 3 days) and look on what has happened in 2017 (bull run) and 2018 (bearish).