Maybe people who got the game early get screwed over?
Nah, I joke. Maybe it's a set thing. Like the game only recognizes 3 months after it's initial release. Could be programmed so that everyone has to wait to get it done at once. I hope it's not glitched
I unlocked and beat the final mission within 4 hours, and I took a lot of free time running around doing random stuff. By the time I finish up the collectibles and get gold on all the missions, it'll probably be about 8-10 hours. The challenges based achievement might take a while though.