It depends on a lot of things: your size, if you have put on / lost weight, how deep the piercing is, how high it is etc. These type of piercings will naturally grow out eventually because they are always pushing against the natural lie of the skin (eyebrow, neck and chest piercings are the same). The only places you can get pierced that will stay open are ears, nose and lip. The shortest-lived piercing is usually the back of the neck, followed by the eyebrow, and then the bellybutton, and while 2 and a half years is quite a short time, it is just one of those things. I had a friend who's bellybutton piercing came out after six months.