As far as I know, it took 3-4 months before the market turns green after the bitcoin halving in 2016. So if history repeats itself then perhaps 2020 will be the year wherein we might see another 2017 in the making.
The devil is in paying particular attention to details. Let's say for a start that prices had been rising since the beginning of 2016 (actually, since September, 2015), and just a few weeks before the halving many people predictably decided to cash out with prices going down the hill. Well, it may not be them in fact as it may just have been some whale forerunning everyone else in the sell-off, but there we were.
Then the Bitfinex hack happened on August 2nd with over 120k bitcoins stolen, which sent the prices further down. So no surprise that the recovery took a few months. The same sequence of events is unlikely to repeat itself, and the next time things may evolve in a quite different direction.