Unfortunately 25 months already, so quite a bit higher than 16. I really hope it won't be long now, I'm worried about the sales after such a long wait, peoples just forget to check.
We got something similar with Baccano! between the second to last and the last currently published book, with a bit more than 2 years, and I don't thin k the sales were any good (well, baccano never sold well anyway). Difference is rokka still probably has more volume to go than baccano, so you don't want to delay that for too long...
Volume 5 should logically be released mid August.
If you look at the "about" part of the profile here :
https://myanimelist.net/profile/Zefyris
you can have (almost, since there's one or two series I have yet to put there) the full list (hit the arrow, then hit the spoiler buttong if you want everything).
To answer directly, right now the novel I'm the most enthusiast about every times a new volume is released is Honzuki no gekokujou. I'm a big fan and every times I finish a volume I can't wait for the next one. Luckily, there's one new every 3 months so the wait isn't so long.