When How Long for and Since?

We often use for and since when talking about time. for + period: a "period" is a duration of time - five minutes, two weeks, six years. For means "from the beginning of the period to the end of the period". since + point: a "point" is a precise moment in time - 9 o'clock, 1st January, Monday.

When should we use since and for?

Remember, for is used with a period of time. Since is used to refer to a specific point in time. You can use for and since with similar verb tenses, if you wish.

What is the difference between how long and since when?

How long have you been here? - how long is the period during which you've been here? Since when have you been here? - when did the period during which you've been here start? In both cases the tense tells us that you are still here. An example should be clearer.