If you study a lot, a year maybe. At least to get conversational. Three to four years to get fluent.

It took me only a year to speak fluent Swedish and understand Swedish people. And I have never been to Sweden, just talked to Swedish people as much as possible and read a lot of Swedish blogs and different texts about everyday life. Man, you live in Sweden. You can get as much practice as you wish! You can definitely make it in a year or even less! First, remember the grammar rules, they are pretty easy compared to English grammar. Then, expand your vocabulary. And here you go, your Swedish is getting better and better! Lycka till! :)

I think the answer totally depends on yourself. How much effort are you gonna put on learning Swedish. The more time you put, the faster you master the languages. It's just the way it is. Perhaps, you could keep staying in an Swedish speaking environment by making more Swedish friends. Then you are good to go!

English grammar and Swedish grammar aren't that different(at least not when compared to Chinese), so knowing English will certainly help you.

I don't think it will take you long to learn Swedish. It is much easier for German people to learn it because there is a great number of the same or similar words in both languages. But nevertheless Swedish is another language and you will have to work hard to learn it fast. Good luck!