j'en ai plus qu'assez

Question

Why do we say j'en ai .... When we also have "de cette période".  Why use the pronoun en when the thing we are replacing is still there.  Eg j'ai plus qu'assez de cette période. In english it sounds like , I have had more than enough of it, this period of..... Is that correct ?

Maarten Kamp · Accepted Answer

Andy, this is a fixed expression -  often these expressions do not translate well, or at least not directly.
 ‘ en ‘ can be found idiomatically in a number of French expressions without obvious reason or meaning. 
See link to Laura Lawless discussion below, from which the following is copied ( bolding added ) 
      “ Though it seems redundant, the following expressions can include en as well as de + noun.
c’en est fait de that’s the end of
 
en avoir assez (de) to have had enough (of)
 
 
en avoir marre (de)(familiar) to be fed up (with)
en avoir ras le bol (de)(informal) to have had it up to here (with)
qu’en est il de  what of, what about “ 
https://www.lawlessfrench.com/expressions/adverbial-en-expressions/