How many years would it take to be fully fluent, on average?
Fluent, as in being able to verbally communicate? A year or two of full-time studying and/or immersion.
Being able to read and write like a native, considering how formal written Chinese has the literary denseness of Romantic and Victorian era English (thaaanks, public education system, for emaciating our language)? Quite a few years.
Being able to be indistinguishable from a native speaker, ex. including in pronunciation and nuanced diction? Well, unless your age is still in single digits right now, good luck.
I don't mean to discourage you. I'm only giving you a reality check, because, honestly, I would never bother learning Chinese if I had to learn the language from scratch.
it depends (obviously) on the amount of time you spend on it, your native language and your level of immersion. the foreign service institute says it should take just under two years.
chinese grammar is pretty similar to english in some parts, and for native english speakers the hardest bit is the characters, i've heard.