Since 5.0 Google seem to have settled into a pattern: major versions are released annually in the late Summer/early Autumn, with 7 and 8 both being late August, and the first digit is incremented when this happens. So I think it's a safe bet that Popsicle/Pecan Pie/Punji Stick or whatever will be announced between August and October 2018, with August most likely. It's also a safe bet that the codename will not be one of those things...
(Prior to that it was a bit of a mess: the confectionary codename changed with 1.5, 1.6, 2.0, 2.2, 2.3, 4.0, 4.1 and 4.4, and the intervals between them were erratic too. And that's not counting Honeycomb, which was tablet-only.)
