There are two essential prepositions for talking about how long something has been happening with the present perfect (or present perfect continuous) tense. For and since are often confused or used incorrectly, however, so let’s do a quick clarification!

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The preposition since can only be used to reference a point in time, NOT a duration. So you can say since 2001, since September, since last summer, or since Tuesday, but NOT since five days.

Tom and I have been working together on Rachel's English since two thousand twelve.

Caption 4, Exercises - Tongue Flexibility and the N [n] Sound

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In fact, since nineteen sixty-nine, fifteen other rare and endangered species have also been rescued from the brink.

Captions 50-51, BBC Planet Wild - Alien Animals - Part 5

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For, on the other hand, refers to a duration. It doesn’t matter if something has been happening for 20 minutes or for 20 years. 

We've been doing freestyle for a couple of weeks.

Caption 25, Kiteboarding - Sam Light Interview

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I have been working at the company Phonez and More for several months now.

Caption 1, Business English - Difficulties with coworkers and contracts - Part 1

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I've been on this boat for twenty-two years.

Caption 3, Aqua Quest - Boo Boo

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While for can also be used with the simple past tense or future tense, since is always a clear indicator of the present perfect or present perfect continuous (See this newsletter for more information!). 

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Further Learning
On Yabla English, for and since can be found in most videos! There is even one video in which a famous actor actually misuses the word since, which is indicated in the captions with sic (sic erat scriptum, Latin for "thus was it written"). Can you find it?