Well, personally I'd say they need to stay on the ball where we can't afford another 7 year gap where they need to keep the franchise fresh. DoA5 got released in 2012, DoA5U in 2013 & DoA5LR in 2015 (DoA3X in 2016). That said they are basically the same game with expansions. In other words them releasing it in such a manner isn't a problem, but they'd need to keep delivering new stuff and/or release a newer version within the 2 next years in order to keep the franchise on everybody's lips. Also, only DoA3X is made for the PS4 with current gen tech, where DoA5 in all honesty is a PS3 game. As a result we quite frankly do not have a DoA fighting game on the current gen, where the LR fighters are split between old and new consoles as well. As more fighters get current gen exclusive fighters, the more DoA will falter towards the competition.
For this reason I think they are forced to release an (early) version (kinda like DoA5) sometime in 2017, or at the very least 2018, where they once again release expansions with more content in the years to come.
That early year gap is left with all the cards out that DOA6 will be no different than DOA5 in terms of complaints and everything else. 2017 is far too early to take the necessary plans to just go "Alright, we should start DOA6 now". Simply because the initial plan is rushed and the extra doubt of Team NINJA having ideas with how DOA6 would be, but difficult to actually take them into consideration with all these projects of games they've been working on along with partnerships and co-developing other games. They've been constantly throwing out DLC for the latest one and not further hinting of any plans of a sequel to the game.
Forcing a release now will please the crowd but only temporarily because of the effort put into the game (because it's new obviously, but not carefully going into the deep content of getting the best stuff/fresh out of totally new compared to DOA5 when it was first released) . The best time I think they should release a DOA6 (or at least working on it) is to make it when they don't have anything to do and placed on a higher focus. DOA6 should be the only game they should work on and I'd say the best time to do it is when they don't got any plans with the other games or co-developing games (Square Enix etc.), Also to mention that DOA5 has received support from tournaments and the Battle Royale etc. DOA5 simply has the content to keep it running even if you've seen it multiple times, but the content itself keeps the game alive being presented everywhere. DOA5 by itself still continues to draw the crowd now.
This is what I mean:
2017 = Forced, they listened to the fans of the game far too deeply and simply catering when they don't have to. People can't be serious here.
2018 = Most likely still working on other games, DOA6 will probably be talked about. I can definitely believe a teaser in this year to let people know that DOA6 is a go and a real thing.
2019 = Seems like a good time, NiOh is probably way out by then and probably done with the arcades for Square Enix's Dissidia. I don't see why not. Reasonable and a big surprise.
2020 = Seems a bit too far, don't like it but the more time the better. Reason being is that they don't have an excuse to fail here. The wait will wound me, but the end results is what matters. If a game truly took this long to finish, fuck it....at least it turned out good if not better than all the DOAs because it "took it's time."
I doubt Team NINJA would pull another 7 years, but you definitely want to give them time to make the next build turn out good. Definitely not 7 years though, I would of given up DOA if it took more than 6+ years to make another. Anything pass 2021+ we have a problem.