How long did you sleep last night?你昨晚睡了几个小时?
When did you start class today?你今天什么时候开始上课的?
how long have you been in class today?你在教室（上课）多久了?
How long has Alison been skating?艾莉森滑冰滑了多久了?（还在持续当中的动作）
she's been skating for five hours.她已经滑了5个小时了.
How long did Sam skate?萨姆滑了有多久?（发生在过去,已经完成的动作）
He skated for four hours.他滑了四个小时.