I'm no expert but I think it should be announced at least next year and come out in christmas of 2019 or during summer of 2020.
If you take a look in history at Sony's patterns and behaviours, you'll notice that when at least looking at the Ps3's history, they released 2 models aside from the original ps3 that game out in 2006. And ps4 has already reached that point... So in my opinion, we're a short road ahead from ps5 and the next generation of consoles in general terms of speaking.
So basically it's right around the corner... The corner of more than 6 months away. It's up to Microsoft or Sony to actually lead the market there, though. Both consoles have gotten extra modes and have had at least a lifespan of 4 years, but I do still consider they have some life left in them.
I'm thinking maybe we'll get Ps5 and or Xbox 1080/ TWO (?) announcements around mid 2019 but I definitely feel they're gonna be dropped onto the market at 2020, no doubt about that.
Hope that answers your question!