No one knows. People are hoping soon, but honestly I think the earliest I think we'd get it is the towards the end of fall / winter of 2019, perhaps beginning of 2020.
It's only one attraction, but it took the other attractions forever to get EP. While I think they will not wait as long since it is just an attraction and not a whole section, I think it's going to take awhile for the wait to die down. Not only you have the opening hype and summer, but GE at Disney will further boost attendance in towards the end of August I would imagine will take over September too (even with kids going back to school), and you have the take the general popularity of HP as a whole.
So to me late fall / winter makes the most sense (at the earliest). I say perhaps the beginning of 2020, because then there is the holiday season and peak time to consider. As that boosts wait times in general and would definitely boost even more for something new like Hagrid's coaster.