Board Thread:FAQs/@comment-74.104.151.111-20160128213142/@comment-24198837-20160128213855

RoosterTeeth said a while ago that they had ~10 seasons of story planned and that they "would continue making the series as long as people keep watching it" - or something to that effect.

Keep in mind though - all (or most) of season three was supposed to be in season two. So it's likely they underestimated the length of already planned content as well.