It's always been every four years since 2001.
There was a special 100th birthday one in US in 2016 hence it was played 2015 and that year.
I believe they want to change it so it's played in same summer as euros so 2020 will be first new date that will start the 4 year cycle.
What's the point of having it in the same years as the Euros? I mean, actually no federation has an interest in competing for viewers of the other continent. I always found it refreshing that the Copa was in different years so you could wstch it without missing out on other games. Now I'll have to choose.