how long是对时间长度（时间段）提问题,意为“（历时）多长时间”,
可用for two hours,for one day等来回答.
how soon 是对时间位置提问题,意为“过多久之后”,
可用in two years / weeks / days / hours,at once等来回答.
how often是对时间频率提问题,意为“多长时间一次”,
可用always,seldom,usually,every two days等来回答.如：
How soon can you finish the drawing?过多久你才能画完你的画?
How long have you been to this city?你来这座城市多长时间了?
How often do you go to the museum?你多长时间去一次博物馆?