Tribunal came out six months after Morrowind was released. Bloodmoon came out seven months later (or one year and one month after Morrowind was released). Oblivion's only expansion, Shivering Isles, came out 11 months after Oblivion was released.
Obviously with the releases you've been through you have a better idea, but I reckon they might get them out earlier especially with how many units this has sold compared to previous titles. They'll want to milk the fan base and new players for money fast before they lose interest.
ps. holy [censored] 17,649 posts
