1. It's been two years since I quit smoking. 2. It's been two years since I finished working at that company. You're using the wrong phrases and expressions. You need an action that takes a moment (or thereabouts) of time only. Thus: It's been two years since I've worked there....is wrong.

The way those sentences are phrased is very confusing... "It has been two years since I have worked in the company" and "It has been two years since I have smoked" aren't right. Even if the grammar is correct, neither of them really make sense. Are you trying to say that you've been working at the company since two years ago? If so, "it's been two years since I have worked in the company" is not a good or concise way to say it, since the meaning can be misunderstood by the listener. Instead, it would be good to say "I have worked in the company since two years ago" or something similar. Putting the "have worked" after the "since" in the sentence puts the time frame of the working into question, making the phrase ambiguous.

As per your private request for me to answer these questions, Your sentence, "It HAS been 2 years since I HAVE smoked" is perfectly correct in grammar. However it is WRONG in the way you want it to mean. There is a subltle implication here. For two years, you did not smoke but now you are smoking again (maybe you have cheated etc). For the sentence to concur with your semantics and meaning, then you should have written it as, "It HAD been 2 years since I HAD smoked". Therefore this answers your question about the difference between the two sentences you gave as example, 1. It HAS been 2 years since I HAVE smoked. 2. It HAS been 2 years since I HAVE worked in the company. #2 means that you are still working for the same company and it has been 2 years already. #1 means that it has been 2 years since you last smoked and is starting the bad habit again. If you used "had" instead of "have", this implies that you are now no longer with the same company. As for these last two sentences, 1. It has been 2 years since I SMOKED. 2. It has been 2 years since HAVE SMOKED. #1 is OKAY. The present tense you used implies that at this moment in the present you are talking about a past incident. #2 is grammatically wrong, your left out the word, "I" and the wrong use of present tense of "have". It should be "had". It can also be rephrased as, "It HAS been 2 years since I HAVE stopped smoking." "It has been two years since I had smoked". This implies that you are talking in the present about your past decision not to smoke. You can also say, "It had been two years since I had smoked". This indicates you are referring to a past comment of a past decision. Next time, post your question as a notebook entry. It is easier to use different colors to highlight the changes, emphasis and also to correct your errors in your questions etc.

Since or For is explained clearly here and there is a multiple choice online grammar quiz to help you check your knowledge. Hope it helps! http://www.stgeorges.co.uk/blog/since-or-for-online-test-check-your-english-grammar/