I assume that this is an idiomatice phrase Italian? If so I find it rather confusing at this stage in the course, where we are trying to learn and follow gramatical rules, and suddnely DL seems to depart from them. While this might be a useful phrase if the service for pizza is poor, it doesn't really help me learn the rules of the language!
I think that Italian might not have this fine distinction, so both meaning might be correct (you're in a middle of a time-defined wait, or at the end of a very long wait) and the difference would be guessed by the context.
When I'm lazy, I answer this with a "I wait for a decade" and it is also accepted. Duolingo is fairly benevolent with English tenses, it seems; probably because this fine distinction in tenses isn't available in other languages.
I suspect that "da" in Italian is used in the same way as "depuis" in French. Where in English we use for + a period of time (for a week) and since + an exact point in time (since last Tuesday) depuis is used in both instances in French and I think da is the same. Can anyone confirm this?