H2 2020
P.S. If it peaks this year, then it drops, but sells still enough. If it peaks next year, or this year, but followed by a 2018 with little less sales, then it could happen earlier, and lifetime sales would be higher too.
Everything assuming Sony won't rush PS5 launch, and it shouldn't, as if the market will show that it wants a Hybrid also from Sony next gen, it wouldn't be possible to offer new features, low power consumption, good VR support and a performance increase when docked compared to PS4 base model before 2019.