For at least a few years now.
I remember watching a Hickey talk where I think I recall him saying it was even before 2011, but the exact name escapes me at the moment. However, I was able to find this snippet from 2011:
Fogus: Clojure was once in parallel development on both the JVM and the CLR, why did you eventually decide to focus in on the former?
Hickey: I got tired of doing everything twice, and wanted instead to do twice as much.