You see how long you are in the air and the whole time you are in the air, you are traveling horizontally at this speed.

你可以知道在空中停留的时间,当你在空中运动的时候,你一直以这个水平速度在运动

耶鲁公开课 - 基础物理课程节选

How long" can be used in a question and it refers to a length of time or a length of distance.

How long” 句型用于问句中，用来询问时间或距离的长短。

How long课堂 - SpeakingMax英语口语达人