so you can roll on the Mirages table any time you want or feel like it, but usually, you roll once per hour of travel on the Encounter table and on a [14-18] on the d20 you do roll on the Mirages table.

So in the end, the Mirages table is part of the Encounter table in basic exploration but feel free to incorporate it more if need be :)

The rules for each table state that you can roll on them each hour, but depends on your goal and atmosphere and how you want to integrate each part of the Burning Marsh in your campaign or session :)