I'm hoping we will get (at the very least) a red carpet soundbite at the premiere of her husband's upcoming film 'Jay Kelly' (in which Greta Gerwig features) at the Venice Film Festival on August 28th.

Given the timing of the festival, and given the prestige of Venice, coupled with the stature that you mention the studio executives seem keen to place her Narnia movie within, Venice also feels like the perfect opportunity for a proper sit-down 'announcement' interview.

Unfortunately, I very much doubt that we’ll get a sit-down interview about Narnia from Greta Gerwig at the Venice Film Festival later this month because I’m confident she won’t want to do anything at all to take the spotlight away from her husband and his film on the day ‘Jay Kelly’ premieres. Maybe we’ll get a bland, newsless, throw away comment while she’s on the red carpet, but I’ll be very surprised if we find out anything substantive from her on the 28th.

My impression is that it’s extremely rare for a movie Director to ever give a sit-down interview before editing, let alone filming, is finished. It’s likely there will be a press release when casting is completed, but my best guess is that we won’t hear much of anything directly from Gerwig herself until next year.  And yes, it’s very frustrating not to know more NOW.

Perhaps a full interview at Venice is a bit much, but I am still expecting them to do some sort of announcement / press release prior.to.the start of filming, which looks likely to be before the end of the August.now.

Therefore, given that the producers seem very eager to talk about GG's movie in the highest loftiest terms, then doing your announcement at Venice would certainly solidify that association. It's a long shot, but definitely a milestone to consider.

Another milestone to put in your diary would be Thanksgiving this year.

IMAX recently made a big deal out of selling-out tickets for Christopher Nolan's The Odyssey a full year in advance of it's release. This is part of IMAX's drive to establish premium format cinema as a highly prized 'book in advance' event, like a concert or stage show. The Odyssey ticket release was also accompanied by a short teaser trailer and a poster reveal.

Therefore, given that I expect IMAX will want to repeat the ticket sellout trick for their big Narnia investment, then they absolutely will need to have done some sort of formal press announcement / marketing prior to the one year mark, otherwise i can't imagine people will be keen to buy tickets for what is still officially "Untitled Narnia Film"

Now that it seems they really are officially getting ready to start filming, I hope there'll be a full interview soon with Gerwig or anyone else who's heavily involved with the project. And (like most of us) I'm almost desperate to have a full cast list, especially who's going to be playing Aslan...  

I think the best we can hope for this year is a press-junket "we're excited about filming" type comment like @rachel suggested. We were hoping to get a comment from Gerwig during a film screening Q&A for the 10th anniversary of Mistress America, but unfortunately, it didn't happen. 

I'm sure we're going to get at least five more generic shoutouts from Rich Gelfond and Ted Sarandos before we get Gerwig talking about her Narnia movie again