This was a little while back:
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On top of this, we would like to note that we plan on extending PTR testing timelines prior to each new season going forward. This will result in a slightly extended season, and will allow for 2 weeks of PTR testing for the community along with an additional week for the team to implement and work feedback/changes in before the season begins. The intention here is to make sure we can apply as much feedback as possible from the community during these PTR timelines while also giving us a good amount of time to test these changes before they go live.
However it was only fairly recently stated that the next D4 quarterly update will come before the next patch notes.