If you are making a statement about something that has been going on for a certain duration and is still continuing at the present time then you always use depuis with present tense. This is different than how you'd say it in English, which looks at the whole period of time from the past, where the action started.

Je suis en route depuis 2 heures. -- I've been on the road for 2 hours.

The case is different if you're not speaking about something that is still ongoing. For example, in reported speech:

Il m'a dit qu'il avait été en route depuis 2 heures. -- He told me that he had been on the route for 2 hours.

In French, you always look at the interval from the current perspective. That's why, in the following sentence, you use imperfect while in English you use past perfect tense (looking at the duration from its beginning in the past).

Je lisais un livre depuis 2 heures quand tu es rentrée. -- I had been reading a book for 2 hours when you came home.

"Vous avez grossi depuis l’année dernière.."

That example from Céline is very interesting, but (understandably) seems directly to contradict the very definite title at the top of this learning module 

 I guess "since" and "for" in English cover a lot of situations that are better differentiated in French.