One of the most common mistakes that native German speakers make when translating the preposition seit to English is to always render it as "since," but in contexts where the English word is meant to be used to indicate the passing of a period of time, it should usually be translated as "for." The mistake seems to be common with Europeans speaking English, as I hear it regularly on the streets and in cafés and restaurants in Berlin, where so many Europeans are speaking English as a lingua franca.

BANNER PLACEHOLDER

Yes, I have been going there since (correct: for) many years.
We've been wanting to visit Paris since (correct: for) at least a decade.
I have lived in Berlin since (correct: for) 16 years. 

It's easy to see how this mistake is made, as the German sentence uses the word seit and as it's actually correct to use "since" if a specific year is used in the same sentences: 

Yes, I have been going there since 2018.
We've been wanting to visit Paris since at least 2012.
I have lived in Berlin since 2006. 

Here are some examples of seit used on Yabla German with the correct English translations "for" and "since." Note the way that the context of each sentence changes which English word is used.

„Mir ist so“, maulte Piggeldy, „als tue ich das schon seit Tagen!“

"For me it's like," muttered Piggeldy, "as if I have already been doing this for days!"

Caption 11, Piggeldy und Frederick - Wanderdüne

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Seit Montag wird das Lied im Radio gespielt.

The song has been playing on the radio since Monday.

Caption 33, Liza - Ein Lied für Opel

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Sie schreibt ihr Referat, an dem sie schon seit zwei Wochen arbeitet.

She is writing her paper that she has been working on for two weeks.

Caption 15, Deutschkurs in Blaubeuren - Relativsätze mit Präpositionen

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Seit vergangener Woche hat die Kranich-Airline nun ihren ersten gebrandeten A dreihundertachtzig in Betrieb.

Since last week, the crane airline has now had its first branded A three hundred and eighty at work.

Caption 9, Rund um den Flughafen - Der neue Airbus A-380

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Na ja, ich krieg' ja seit Monaten keine Wochenendzeitungen mehr.

Oh well, I haven't been getting a weekend newspaper for months.

Caption 8, Großstadtrevier - Von Monstern und Mördern

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OK, ihr seid jetzt seit April in Deutschland, ja?

OK, you have been in Germany now since April, right?

Caption 1, Deutschkurs in Blaubeuren - Relativsätze mit Präpositionen

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Ich spiel' jetzt schon seit zehn Jahren Handball.

I've been playing handball for ten years now.

Caption 7, Jenny und Alena - Handball

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Also, die Nachhaltigkeitsstrategie gibt es seit dem Jahr zweitausend-und-acht.

Well, the sustainability strategy has been there since the year two thousand eight.

Caption 37, Rheinmain im Blick - Nachhaltigkeit

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A general rule of thumb is to translate seit as "since" if it refers to a specific date such as a year, month, or day of the week. If it is referring to a length of time, such as "many years," then seit is usually translated as "for."

Further Learning
Compare other contexts in which seit is used on Yabla German and study some other uses of the word in which the preposition seit can also be translated as "because."