I reckon it's easily a year away...
Consoles tend to release just before Christmas, so if I was a betting man, I'd probably say the Playstation 5 will be released Christmas 2019 in the US. I think this is very likely because they'll want to get ahead of the XBox 2 (apparently launching early 2020).
I've had a Playstation 4 for 5 years now, and I have to say that I still think it's absolutely amazing... but Sony has stated that it's at the end of it's lifecycle. Sony are expecting global sales 16 million consoles this year, so they'll be looking to inject some new enthusiasm into their market.
It's hard for me to imagine just how much better the PS5 could be than the PS4 for me specifically. I only buy a couple of games a year, and then play the absolute heck out of them. I download those games onto my physical PS4 harddrive.
From everything I've read, the PS5 will be more of a streaming game platform... which doesn't work for me personally because my internet is terrible. This means that people would 'rent' games for a limited time... a day, a week, etc.
It's an interesting model... at the moment, please pay say $100 for a game... a small percentage of those people get amazing value out of it.. while most play it for a little while and then it sits on their shelf... a streaming service would mean that heavy users pay a lot more and casual users pay a lot less... the opposite of the current pay models. I'm extremely curious to see how that will go.
I'm also pumped to see just how good the graphics can get on the PS5... after all, the PS4 is pretty amazing, and it'll have to be significant for people to buy in.