depuis v. il y a...que, ça fait...que, voilà...que

depuis

To indicate the starting date or duration of an action or situation which is still going on in the present, use the preposition depuis + a time expression. 

Note that depuis can mean two different things in English. When depuis is followed by a length of time, as in the first example below, it indicates duration and is translated into English by 'for'. When depuis is followed by a date or a point in time, as in the second example, it indicates the starting point and is translated by 'since'.

*Spanish speakers!  Note the difference between depuis and después.

In a question, depuis is generally translated as 'how long' or 'since when'. In most cases, such questions with depuis can  be answered with either the expression of duration or the beginning point. 

In negative sentences, depending on the meaning, depuis may be used with either the present (action still going on) or the passé composé to indicate how long something has not been going on (non-action).

il y a ... que, ça fait ... que, voilà ... que