Likely 2-4 years rough guess. They have too many projects for a DOA game plus the contract with Square's Dissidia for years to come. You can't be surprised if DOA is not for another 3 years.
That is until people get bored of Tekken, like with TTT2. DB is right that TN has too much on their plate right now to develop a full-on DOA6 to a substantial standard.
Team NINJA's goal isn't trying to compete with Tekken in terms of better games (for now). They make the games out of business. They are not going to suddenly say "Tekken 7 will be out in a few more months so it's time for us to make a game." - They are not that crazy to waste funds to challenge a game series from Namco.
"We are not slaves, we are game designers! - We make the games because it is fun. We do not make games to compete with another, we don't want people to get the impression of NiOh's goal to compete with Dark Souls because we have same idea from Dark Souls too, if the game is too hard, it is because it is different and we want it different."
^ That same text can apply to DOA even if it isn't NiOh. You shouldn't make a game right away instantly based on what other companies are doing with directions, that is basically going desperate in being relevant when their goal wasn't to be competitive with another company to begin with. Only the Fighting games itself is competitive, not company businesses in making a fighting game.