I think it'll be longer, and honestly, I'm not particularly looking forward to it. SM seemed long to me, as someone who's played every Pokemon game up to this point at least twice. To a first-time player, I'm relatively sure the games felt even longer. In fact, I lent my Sun copy to a friend in my computer graphics group as her first Pokemon game ever, and after three months before I left campus I got it back and she'd only beaten the first trial. She even went as far to say it 'slogged on' a bit at the beginning.
Hopefully it's different in USUM, but as a potential sequel, it's likely going to be longer.