How long will it take me to learn Chinese as a native English speaker?

That depends on your foreign language aptitude, your motivation, and the time available.

That is as indefinite as asking how long it would take to learn to drive. Different people would take different times.

According to the Foreign Service Institute (FSI) of the US Department of State, about 2200 hours, with about half that time preferably spent studying in a Chinese-speaking community if you want to be fluent.

Depends on how much effort you put into it.

The fastest way to learn any language is live in that country while attending language school.

The combination of formal training plus the day-to-day activities will force you to use your training.

Otherwise, you can get a decent background with 2 years of classes. However all the classes in the world won't be very effective unless you actively use your language skills outside of class. This involves practicing your language, preferably with native speakers. Watching TV shows, movies, or listening the radio or music in that language. When you get a bit more advanced, reading things in that language is also a must. This can be newspapers, comics, novels, etc.

Coming from English, you will find Chinese grammar a lot easier. There are no verb tenses in Chinese. Word order is very fixed.

On the difficult side, Chinese has a few sounds that don't really exist in English, and several them sound similar (e.g. zhi, shi, si) Also, Chinese is a tonal language. This means sound+tone gives the meaning. Even if you use the same sound, but alter the tone, you will alter the meaning. For example, "ma" can mean "mother", "horse", or is used to mark a sentence as a question. (and yes, "ma ma ma?" could mean "Mother horse?")

Then there are the Chinese characters themselves. If you do take a class, be sure it incorporates Chinese characters with the material. Once you get an idea of how you use these characters to make sentences, other aspects of the spoken language start to make more sense. While it may seem daunting at first, it's really not that bad since you are introduced to the most common characters first, and you'll learn to recognize them through sheer repetition. Also, you'll learn to recognize how characters are often made up of common components, usually one component for general meaning, and one component for sound. For instance the character for mother has the component for woman, and the component for horse. This allows you to make some guesses about characters you don't recognize.

Learning a foreign language is not easy, you have to be really passionate. Watch a lot of stuff in Chinese. I started English as a child, I watched a lot of English speaking channels, but still, I sound like a retard sometimes, especially because my accent is naturally American when I speak English, and I make a lot of grammatical mistakes.

Stick to English.