I will come back soon.
A: David, how long is your visit to Beijing?
A: 大卫，你在北京待多长时间？
B: Just two weeks, unfortunately. But I will come back soon.
B: 只有两周，很遗憾。但是我很快就会回来。
A: I'm sure you wil. When are you leaving?
A: 我相信你很快就会回来的。你什么时候动身？
B: I'm leaving on the 15th.
B: 我15号走。
A: So soon. That's next week.
A: 很快了。就是下周。
B: Yes, but I'll be back again next month.
B: 是的，但是我下个月还会回来。
Notes:
1. To ask about how long something is, you begin with How long...? e.g. How long is your vist to Beijing; How long is the flight to Shanghai? 要问某件事情有多长时间，你这样开始：How long..? (...多长时间？)例如：你在北京待多长时间？到上海的航班需多长时间？
2. To ask when something is happening, use When...e.g. When are you leaving; When are you arriving? 要问某事什么时候发生，用when..? (什么时候...?)。例如：你什么时候走？/你什么时候到达？