We had not been out for long, when she felt sick.此句为什么when,用as行不行？

不能用AS因为：
as作“在-------期间，当----的时候”引导时间状语从句.注意与when、 while的用法区别。
①下列情形时,只用as, 而不用when或while。
1)用于表示同一个人的两种动作交替进行,指一边----一边. 如:
The girl sings as she goes to school.
He looked behind occasionally as he went forward in the forest.
2)表示两个同步发生的动作或行为,意思是随着-----的发展.如:
As time went on / by, she became more and more beautiful .
As children get older, they become more and more interested in everything.
3)表示两个短暂行为或事情几乎同时发生.如:I watched her as she read the book.
I thought of it just as you opened your mouth.
Just as the flying worm hit her face, she gave a loud cry.
4)接在名词后面表示某一个年龄段时.如:
As a young man, he was active in sports.
②when作"当……的时候"解,可以指较短的(一点)时间,也可指一段时间。如:When I was at college, I could speak several foreign languages. When the clock struck twelve, I had slept for 2hours.
③ while常表示一段较长的时间或一个过程,强调主句谓语动词与从句谓语动词同时发生。如:Work while you work. Play while you play.

表时间啊！不能换
我们没出去多久，她就感觉不