In Russian we use the expression сколько времени?
to ask "for how long" something takes place.
This question also means "What time is it?"
The answer to this question is always in the accusative case:
--Сколько времени вы читали? How long were you reading?
--Весь день. Всью субботу. Всё утро. All day. All Saturday. All morning.
There are two Russian words that are used to express duration of
time, and they are sometimes confusing for English speakers. They are
долго and давно. They both mean "a long time" or "for a long time".
The word долго is used to indicate the length of time an action
took place or will take place:
Мы долго делали упражнение. We were doing the exercise for a long time.
Они долго работали в Москве. They worked in Moscow for a long time.
The word давно is used to indicate an action that began in the
past and is going on in the present. It is equivalent to the English
expression "(since) a long time ago":
Вы давно работаете здесь? Have you been woirking here for a long time?
It can also be used to indicate that an action took place a long
time ago:
Этот ресторан построили уже давно. They built this restaurant a long time ago.