Re: How long should the new series go on for?
No ending. There should be a jump at some point when all the JL films and Bruce as Batman solos are out of ideas...and they just go into Batman Beyond. It could be the same Bruce re-cast, or ignore the continuity. Do it so it's whatever we want it to be.
Besides JL, I think it should go past a trilogy while Bruce is Batman. By the third film they should start building the bat-family. There's enough rogues to keep going to a possible double trilogy. Different directors, etc.