depuis + imparfait

The preposition depuis with the imperfect indicates how long something had been going on in relation to a past event. Basic sentence meaning and construction are similar to those of depuis with present. Depuis with imperfect may indicate either duration or a temporal starting point. 

Duration may also be indicated with il y avait ... que, cela faisait ... que and, more emphatic, voilà que. The first sentence above could also be expressed as:

Il y avait ... Cela faisait ... Voilà … une heure que Joachim attendait Kanye. Joachim had been waiting for Kanye for an hour.