以后 means "afterwards"/"later on"[in the future] 我以后要去中国 - I'm going to China in the future. 一会儿 means "in a little bit"(in a few minutes) 我等一会儿带你去看医生- I'll take you to see the doctor in a few. "when" can be translated as “什么时候”. 什么 meaning "what" and “时候" meaning "time/moment" . 他什么时候回来- When is he coming back? 明天下午三点左右- Around three o'clock tomorrow.

you mean sentences like：I'm about to leave the house when the phone rings.? you can use"当(sbj1)vp1的时候，(subj2)vp2" 当我要离开房间的时候，电话响了

the second qustion ,maybe like this ?why do you want a new job when you've got such a good one already?

以后：later\一会儿：a few minutes when-多久

in my opinion, chinese people would like use 一会儿 while it's within 2 hours. but 以后 while it's more than several days even some weeks. 我认为，不超过2小时的时间，中国人会用以后，超过几天甚至几周的时间，才会用以后。 when 以后 is used as "after", it's not limited about of time. you can say "any time 以后". eg. 炸弹会在5秒钟以后爆炸。 the bomb will bomb after 5 seconds. the second qusetion, 在中国（用汉语?）该怎样问“何时”的问题，如果问题跟时间无关？ 不太懂你的意思，你可以英语举个例子。 would you like give us an example in english?