Rêve, a rêve ou rêvait

Question

Why is it “Ça fait longtemps qu’elle en rêve”, in the present tense, and not either le passé composé (en a rêvé) or L’imparfait (en rêvait) ?

Maarten Kamp · Accepted Answer

Einar, 
it is the present tense, not the subjunctive.
The expression “ ça fait … que “ is a very commonly used oral ( informal, conversational ) expression and synonym for ‘ depuis ‘. Depuis remains more common in more formal speech or writing though.
It is used at the start of a sentence, as here. ‘ Ça fait longtemps qu’elle en rêve .. ‘.
 Depuis can move in the sentence and can give slightly different emphasis to either the action -  ‘ Elle en rêve depuis longtemps ‘, or the duration - ‘ Depuis longtemps, elle en rêve’ 
Just as with depuis, ‘ ça fait (duration) que…. ‘ is used in the present tense for something that is happening/true today and to express the duration since commencing. In this example the duration is  ‘ longtemps ‘. 
An old link to an answer from Cécile :
https://french.kwiziq.com/questions/view/elle-est-coincee-la-haut-depuis-plus-de-cinq-heures-please-explain-why-est-coincee-is-in-the-presen

Einar Vikingur · Answer

It just occurred to me that “en rêve” is the same in the present tense and the subjunctive. So is this a construction in the subjunctive, then?