That time limit is stated plainly in the above PlayStation 4 ad. At the 1:19 mark, the ad trumpets "60 minutes of exclusive gameplay" only found in PlayStation 4 version of Assassin's Creed IV, but as the footnote explains this exclusivity is valid for six months. A similar scene focused on Watch Dogs plays out seconds later.
As Shacknews points out, this isn't necessarily a guarantee that these DLC packs will find their way to Microsoft- and Nintendo-branded consoles. Ubisoft could ignore other platforms once the exclusivity agreement ends, though that makes little financial sense if Assassin's Creed IV and Watch Dogs are the big hits Ubisoft intends them to be.