Until it's started? Completed?
How long is it going to go for? (Hours of gameplay? Number of missions)
Since i don't know what your question was aimed at, i'll answer to all.
The campaign has just been started, with only one little cutscene, which has a fair bit to do until it's finished. This is because the team is busy (including myself), and i've been working on other features. It could be a month or more until the next release, and we're planning to have at least 1-2 missions in the next release.
When the storyline is finished, we plan to have a lot of gameplay. Who wants to play a game where it's storyline is an hour long!! Not me! Also, once you finish the campaign in 'Normal', you will unlock something. When you finish it in 'Heroic', you will unlock something else, and the same for legendary. So this will encourage you to finish it in all difficulties.
I hope i answered your question!
Scotty