There isn’t any order to the boxes appearing.
Can check out this thread for people that have actually tracked their data: Elemental Chest Frequency - days tracking and results > estimated between 4 to 16 days
But to show evidence; this is the history by @zephyr1 of 2020 (before he went casual) and 2019:
There is also tracking in that thread by @littleKAF, @jinbatsu, @yelnats_24 and many more. All showing that there is no pattern to the colour box that appears nor the position that the box appears in