devtrackers.gg
11.9 will go to PBE as 11.8 starts rolling out to live servers. Typically this means that you'll get the next patch on PBE sometime on the Tuesday afternoon (PDT) of a new patch cycle. Unlike the live state of the game, there is only one "region" for PBE so it happens all at once.
TL;DR: Reckoning will go to PBE in the afternoon of the 13th PDT. Because it is a test environment, and we're adding a ton of new stuff, the exact timing could vary.