A Practical Guide to German:

Time Phrases, or Wie lange?

From Walter F. W. Lohnes and F. W. Strothmann, German: A Structural Approach (3rd ed.; New York: W. W. Norton, 1980), 153-4.

It's all in the past:

Let us assume that you want to tell somebody that you used to live in Berlin and that you lived there for three years.  In English, you will say:

I lived in Berlin (for) three years. 

Notice that you are reporting on a situation that is all in the past, that you are using the past tense I lived, and that you have to option of expressing the period of duration with or without the preposition for.  In the German equivalent of this sentence, however, you must, as in all conversation referring to the past, use the perfect tense.  Hence, you will say:

Ich habe drei Jahre in Berlin gewohnt. 

Notice, too, that in German the period of duration is expressed with a time phrase in the accusative case and without a preposition.

It started in the past, but continues!

Now let us assume that you are living in Berlin right now and you want to tell someone:

I have been living in Berlin (for) three years. 

Notice that you used the perfect tense . . . to talk about a present situation . . . , and that you again expressed the period of duration with an optional preposition, for. 

A German does things quite differently.  As he is dealing with a present situation, he uses what is to him very logical, the present tense.  He will say:

Ich wohne seit drei Jahren in Berlin.

That is the equivalent of the English sentence above, I have been living in Berlin (for) three years.  Notice that in this situation, in which English uses the perfect tense, German uses the present tense. 

Stretch-of-time (duration) phrases:

In English, the time phrases are identical in the two sentences:

Duration of time expression in English

1.	I lived in Berlin (for) three years.	Closed-end situation
2.	I have been living in Berlin (for) three years.	Open-ended situation

In German these time phrases are not the same.  The German says ...

Duration of time expression in German

1.	Ich habe drei Jahre in Berlin gewohnt.	Closed-end situation
2.	Ich wohne seit drei Jahren in Berlin.	Open-ended situation

German stretch-of-time phrases, phrases used to report on duration (wie lange? ... ) divide into two groups which those sentences illustrate.

Closed periods of time: sentence 1 above talks about a time period that is finished, or closed. This closed period of time may be in the past OR in the future!  Here are examples:

Ich habe drei Jahre in Berlin gewohnt.
I lived in Berlin (for) three years.

Ich bleibe drei Jahre in Berlin.
I'll stay in Berlin (for) three years.

Open-ended periods of time:  sentence 2 above talks about a time period that began in the past, but is now on-going and may continue into the future. These phrases are introduced by seit, schon, or schon seit.  A phrase introduced by seit or schon seit is followed by a dative; a phrase introduced by schon is, however, followed by an accusative.  Hence, seit drei Jahren (dative) and schon seit drei Jahren (dative), but schon drei Jahre (accusative).

Listed ... [in the box below] are some commonly used stretch-of-time phrases.

Exception (Lohnes and Strothmann), 155)
There is one type of situation in which German does use the perfect tense with open-end time phrases, namely when the activity described comes to an end at the moment of speaking:

              So gut wie heute abend habe ich schon lange nicht gegessen.
               For a long time, I have not eaten as well as I am eating this evening.

Adapted from Walter F. W. Lohnes and F. W. Strothmann, German: A Structural Approach (3^rd ed.; New York: W. W. Norton, 1980), 154-5.