Lisp is "cognitively" different from, say, c, Java, and python. It's like learning a new language. How long will it take you to be fluent in Arabic? It takes 5 years. But the best part is that after 5 years, you won't forget it.

I've managed to work for 10 years in Lisp. Then, I had to work in Java. After another 10 years of Java, I took up Clojure very easily.