Obviously there will be a PS5.
If we want to speculate on when it will be released we should at least look at the the previous SONY console releases.
PS1: December 3, 1994
PS2: March 4, 2000 - 63 months
PS3: November 11/17, 2006 (jp/na) - 80 months
PS4: November 2013 (na) - 84 months
Being conservative and giving a wide window range I would expect to see:
PS5: November 2019 - 72 months or November 2020 - 84 months