Not sure about only 5 minutes, but it will most probably only last so long as necessary to setup Barnabas' backstory with his arrival in America as played by Thomas McDonell, with focus then shifting to Depp taking over as Barn and his relationships with Josette and Angelique, his betrayal of Ang (at least in her mind), and his cursing and burial.
Here is my logic as to why I believe it will be relatively short: If we have a movie that is 120 minutes and we know the majority of it takes place in the "present" (I love how we can still legitimately use that reference since it is so close to the actual "present" in the original series
), we are looking at probably 100 minutes of the modern Barnabas story, as that is where the majority of the characters are located in the script. We have 3 to 5 possible (known) characters that can play out the drama in 1772, though I believe it will be closer to 3. And, since that time period is presumably more important than 1752 plot-wise, it will have more screen time. I would actually be willing to give 1772 up to 15 minutes of the whole film, since I believe it is vital to setting up the present story, but can probably be told rather efficiently - especially if the WB plot synopsis is any indication. Since we have no word on a young Josette or a young Anqelique, I envision 1752 to play out relatively quickly, probably focusing on the Collinses moving from England, settling in New England, building the mansion and finding out, somehow, that the curse is still with them. 1752 is obviously important to the story, but it can probably be told in about 5 minutes. I speculate that this will be a point in time when there is some tragic event which helps to shape the Barnabas character and make him into the man we see in 1772. And since Barnabas is the master of Collinwood in 1772, I am assuming that Joshua must be dead or dying at that point. And if it is true that Naomi is uncredited, we can assume her role will be very small. Again, all speculation, but I just thought I'd share some of my logic.
I do find it an odd departure that the curse of Collinwood is inherent. Yes, the Collinses have always been seemingly ''cursed'', but Angelique was always seen as the de facto face of that curse, or at least the main proponent of it from the Barnabas side of things. After all, the reason Barnabas lost everyone he loved was because of Angelique. I wonder if this newly-inserted "official" curse will have any bearing on the curse which Angelique executes. The curse of Barnabas Collins was probably the most tragic part of the original and 1991 series, as its results were far more devastating than just the transformation of Barnabas Collins. The curse reached across the centuries and affected nearly everything Barnabas did and all of his relationships. A new "official" curse on the family would seem to take away from the power of Anqelique's curse, imo. But, we already saw that curse brilliantly (and powerfully) executed in the final episodes of 1991, so I guess it's time to look forward to a new take on that aspect of the story. I guess it's really just going to put Angelique's curse in perspective when pitted against the curse of the whole family.