I believe Mao and Jun are together. I’m not saying this as a shipper, but based on the eyewitnesses’ reports, articles & evidences that I’ve been reading about them. All the marriage rumors for the past 2.5 years points to the 100% possibility of them being a long-term couple with marriage in mind. (what’s funny about this ship is that they haven’t even admitted to dating but the issues surrounding them are already of marriage level) As for the years they have been together, I’m not sure when it started but Hana Yori Dango became the key to their relationship (whether they started dating during or at the end of the drama).
One thing that makes this couple legit is that they never denied the rumors, I mean it’s really tiring to be rumored with the same person for almost 7 years unless something’s going on (and you like it lol). It’s easy to deny and say they’re just good friends, but both sides never said so. The silence itself plays a critical hint on the real answer.
As for marriage it’s likely impossible for it to be this year but who knows? They always surprise us after all lol. With the taiga shooting preoccupying Mao and Jun busy with concerts, tying of the knot is unlikely to happen at the moment (but they can always register their marriage in the city hall so that we can have the real Matsumoto Mao haha). Whatever plans this couple have, I will patiently wait and support them. :D
Anybody is free to argue and say otherwise, but this is what I believed and what the two of them is showing us.