We use the words "since" to talk about the duration of something from a specific point in time. We use "for a long time" for things that have gone on/ continued longer that we expected it to. Or we can use it for things that have simply gone on for a long time. this phrase doesn't really talk about a specific point in time. So here is an example. I was born in the year 1994. "I have been alive in 1994". This is a very strange sentence but I hope you understand the meaning better after reading it. Ex: I have been married for a long time (50 years). Ex: He has been sleeping for a long time (10 hours) Ex. My father has been dead for a long time. (8 years) It's kind of all relative. 10 hours is a long time to sleep because the norm is 7-8 hours. And 50 years is a long time to be married because many people get divorced so often.

Having seen your last question about the translation of 'da tanto tempo', I can see why you are confused about these expressions. Basically, the Italian word 'da' can be translated into English in two ways: 'For' followed by a PERIOD of time. 'Since' followed by a POINT in time. So, for example, you can say: For a long time For ten years For five minutes and so on or Since 3 o'clock Since November Since 2005 Here are some examples with your original sentence: Non vado a pescare da tanto tempo = I haven't been fishing for a long time Non vado a pescare da sei mesi = I haven't been fishing for six months Non vado a pescare da molti anni = I haven't been fishing for many years but Non vado a pescare da settembre = I haven't been fishing since September Non vado a pescare da 2012 = I haven't been fishing since 2012 Non vado a pescare da quando ero bambino = I haven't been fishing since I was a child. I hope that helps. Please ask again if anything's not clear.