Echoing what a PP said above - DH had to get a new one and it was just under 4 weeks. And that one had to have involved some extra work on their end. Long story involving a misreporting of his old passport as stolen...
I think DCL is sort of vague about the 75 days bit. It says they "ask you to" input your passport number, and of course for some sailings people don't even strictly have to use passports. The language doesn't specifically state that they require you to input it 75 days out, unless that is specifically stated on your own reservation? maybe if you're using DCL for air transfers and the airline needs it? Just b/c you *can* check in at 75 days certainly doesn't mean everyone who has a reservation will, so what will DCL do for the people who don't get around to doing online check-in until 74, 73, 60 days out? If cruisers don't care about Palo reservations, m&g tix, or getting an early boarding time, seems like it wouldn't be a high priority to do all that immediately when the window opens. People could book a cruise less than 75 days out, too. Anyway, long-winded, but it seems like that would be a bear to try to enforce.