GraalVM team member here. Implementing any mainstream language is indeed a challenge, more so if you have to maintain bug-compatibility and cope with all the bits of bad design that went through the cracks in the de-facto implementation. Truffle is not for beginners, but knowing the basic set of features e.g. partial evaluation, deoptimization... can get you very far already e.g. you can easily speedup any interpreter by 10X or more with minimal changes.

How long does take to implement a programming language? Well, from hours to years... depending on the language. To make my point; how long would it take to implement a JVM? A JVM is a complex beast, so I would myself guess from years to a decade probably, what if I told you, that Espresso was written in just 6 months by an intern and a seasoned engineer... in just 6 months it was able to Minecraft and even run itself. I assure you there's no magic here, and certainly no blinding talent either; the only reason for this unheard productivity was Graal/Truffle. So, whenever I talk about Espresso I always give all credit to Graal/Truffle, it is a sublime platform for implementing fast languages and runtimes, of which Espresso is just a byproduct.