How to reach the city center from the Xian airport? And how long will it be?

Well, what do you mean by the city center? Do you refer to Bell Tower Area?
If yes, upon landing, you can take shuttle bus line 1, get off at Xishaomen Airport Business Hotel first. It takes around an hour on the way. The ticket fare is CNY 25 per person.
After that, take a taxi to get to Bell Tower. The fare is CNY 10 to 20 in the day time.