I think the length of the next saga will depend on how big the region is, how much time til the next gen games, and everything that's in the region. I wouldn't want them to shorten the region just for another arc if that meant skipping a lot of important things or half-assing some stuff.
Every gap is probably going to be 4 years. They could have easily saved the BW games for the next handheld, but considering they released them on the DS it proves only a 4-year gap will likely happen.
I still don't see why the saga can't be three years + 1 filler arc like they did in AG.
The problem with this structure is Ash's Pokemon and the female character only get one region, whereas May and Ash's Hoenn Pokemon got practically two.