Probably around the end of the month if previous (normal) years are anything to go by. Here are the dates of main lineup announcement from last few years. There are always some added later (including some headliners sometimes), but as you say, the first poster tends to be fairly full:
10 Feb 2022
16 July 2021 (COVID uncertainty)
29 Jan 2020
29 Jan 2019
25 Jan 2018
26 Jan 2017