Following today's Pokémon festivities, Nintendo of Europe sent out a press release confirming details around E3 live streams and the multiple 'Invitational' events. It also touched upon timings for the Nintendo Treehouse live demonstrations, with the wording indicating a runtime of around 30 minutes for the showpiece 'Nintendo Spotlight' broadcast.
Below is the relevant press release excerpt.
On Tuesday, June 13, the show starts right after the Nintendo Spotlight: E3 2017 presentation about 9:30 a.m. PT. On Wednesday and Thursday, June 14-15, the show starts about 10 a.m. PT.*
The Spotlight is due to start at 9am Pacific / noon Eastern / 5pm UK / 6pm CEST on 13th June, which is why that 30 minute estimate is relevant. The asterisk in that quote does reference the fact that times are 'subject to change'.
If the Spotlight is about 30 minutes (and it is still an if) how would that compare to past events? The 'Nintendo Digital Event' broadcasts in 2014 and 2015 were both between 45-50 minutes, while in 2016 we had a world premiere trailer of The Legend of Zelda: Breath of the Wild that then cut straight into demonstrations of the game.
If this is accurate the Spotlight would be relatively short compared to past multi-game showcase presentations, but that may not be a bad thing. We'll find out on 13th June, in any case.