June 10th!
I read the next chapter of Teasing Master Takagi-san today, which was the final chapter of volume 6.
I ordered the rest of the series a few days ago too since I think at this point there is very little likelihood of me not wanting to read them all! 
I got really lucky though, because I put them all into my basket on CDJapan, but then decided not to order them straight away as I have a friend who I sometimes combine orders with so we can share the shipping costs.
Then I went to bed, and woke up the next morning to an email from CDJapan telling me that Volume 18 had just been released! I had no idea it was coming out yet.
I would have been so cross if I ordered โthe rest of the seriesโ and then woke up the next day and there was a new one. 