PlayStation 1 / December 1995 - March 2005
PlayStation 2 / March 2000 - December 2012 // 4 years, 4 months
PlayStation 3 / November 2006 - May 2017 // 6 years, 8 months
PlayStation 4 / November 2013 - ??? 202? // 7 years, 0 months
PS1 was 10 years on the market
PS2 was 12 years on the market
PS3 was 11 years on the market
2020 might be the launch year for PS5, 7 years after PS4 while the latter stays in production till 2023/2025.
The only oddball was PS3 but that's because they were bleeding money, no reason to think with them winning like the other generations that they'll prolong it.
Keep in mind that prior or after a new console is launched Sony's focus will shift to developing countries (India, Brazil, etc etc so they'll still sell buckets of PS4's).
I don't think the console itself matters, it's about locking players in their eco-system, a new console helps with that especially since they are interested in the PC crowd.
Nothing speaks more to such players than a native 4K machine they can get for cheap instead of a $1000-2000 PC, it's all about the games and that's something they always had.
The big money is from services, PS+, games on PSN, they are probably selling PS4 Slim at a loss because revenue from their Network Services off-sets their loss on hardware.