Like 9 months or so, but the dev was pretty clear back then that he wanted to create a big DLC. For this one he doesn't want to do paid DLC, just something "big" with new scenes and things like that.
If anything I remember he said something along the lines of "I've been working on Summer/Winter Memories for so many years now, I wanna finish this and do other stuff".
He gave his fans a beta of a medieval game he was working waaaaay back (before summer memories +) in this year's comiket. I think that's gonna be the next thing after he's done with winter memories.