This paraphrase you mentioned is sometimes what it asks for rather than literal translation. Unless English to Portugese is suppose to be literal whist Portugese to English can be paraphrased. I say this because ive noticed when i literally translate Portugese to English I usually get it wrong or an exact translation makes no sense.
Seems to me this is about an official dinner party not just any other meal, or you wouldn't say do jantar. And you're asking how much time you should plan to spend. If that's the case, DavigiFallon's suggestion How long is the dinner for? works for me; as well as Bblouck's How long is dinner for? Or: how much time will (the) dinner take?
Is this normal phrasing? What I mean is, in American English the common phrasing might be "how long is dinner?" although one could phrase it "what is the duration of dinner?" but it would be uncommon phrasing. Is "qual é a duração do jantar?" common Brazilian phrasing?