I think @alex77034 made a fair point; making the fly key 24 hours longer wouldn’t be bad at all. Usually when buying these, players tend to use them to work on large build projects, not just because they want to fly around. These builds can take a while, and I think two days is fair.
I also agree with @Cooleysworld:
Making it one week would drastically make the value drop, which isn’t great for two reasons, one because players won’t vote as much (as previously said) and they’ll just buy it, and two, because Ultimates won’t be very happy ‘bout that.