The French temporal expressions depuis and il y a have distinctly different meanings and uses, yet they often present difficulties for French students. Here is a detailed explanation and comparison of depuis and il y a to help you clearly understand the difference once and for all.
Depuis, meaning "for" or "since," can be used in the present or past in order to express an action that began in the past and continued to the temporal reference point used in the sentence: either the present or some point in the past.
Depuis is thus used for actions that were incomplete at the referenced time, and can refer to two different kinds of time:
1) When followed by a period of time, depuis indicates the duration of an action and is equivalent to "have been + -ing (perfect progressive) + for"*
Nous attendons depuis une heure.
We've been waiting for an hour.
Il parle depuis 5 minutes.
He's been speaking for 5 minutes.
Il travaillait depuis 10 jours quand je l'ai vu.
He'd been working for 10 days when I saw him.

2) When followed by an event or point in time, depuis indicates the start time of an action and is translated in English by "have + -en/-ed (perfect tense) + since/for"
Je suis malade depuis mon arrivée
I've been sick since I got here.
Il était fâché depuis l'annonce, mais maintenant...
He had been angry since the announcement, but now...
Depuis hier, je suis déprimée.
I've been depressed since yesterday.
Il ne fume pas depuis un an.
He hasn't smoked for a year.