It depends on how intensively you study, whether you study in China or not, etc.
As an example, I currently study Chinese at college under a government funded program that is super intensive, where most students, after 4 years of classroom instruction plus 1 year of studying and working in China are essentially fluent in Chinese in that they can read newspapers/watch news, discuss politics, and even write master's degree theses completely in Chinese.
For reference, regarding my own experience, I'm currently in a 3rd year level class and we use newspaper articles/broadcasts as study material, but because newspapers use highly formal language, it's quite challenging. If I pick up a magazine or newspaper from a store, I might be able to get the general gist of an article.
If you're studying it on your own, if you put in enough time you can also quickly learn it, but if you just study like 20 minutes a day, it'll be a long while before you're even close to fluent. Plus on top of modern Chinese, you have to decide whether or not you want to study Classical Chinese (and if you want to participate in discussions with Chinese people regarding ancient Chinese history and culture, you will want to study it).
Don't fall for the meme that Chinese is impossible, but understand you just need to put in a lot of work to learn it.
Regarding Japanese, as I've studied Japanese in the past I can't really give an accurate description of what it's like to read Japanese with just Chinese knowledge, but a few of my Chinese friends have said if they look at a Japanese news article or something, they might have a very general understanding of what it's about, but due to hiragana, katakana, etc. they can't understand much.
learnnc.org/lp/pages/6427
resources.allsetlearning.com/chinese/pronunciation/
resources.allsetlearning.com/chinese/grammar/
mdbg.net/chindict/chindict.php
gloss.dliflc.edu