If you want to study mandarin, there is no exact answer as to how fast an average person acquires enough skills in the Chinese language to converse/write fluently. Like all the languages of the world, it will depend on your age, your desire to learn, time you spend learning, and other factors. If you just take Chinese lessons and don’t actually put in the time to do some self-studying, it’s going to be a long while.
On average if you want to become fluent in Chinese language, it will take some time, usually if you take a 4 year Chinese courses; it should be sufficient to get you fairly fluent. If you want to just become conversationally fluent, it is highly possible to achieve fluency within 1-2 years if you live in China or regularly surround yourself with Chinese people.

Why do you choose to find a mandarin school? Like M andarin Garden, you can find more details on http://www.mandaringarden.org and has a sample course.