Hi, the loot boxes in the open world are on a 24h respawn timer. So open them as you see them, you have nothing to loose.
One exception are the big orange boxes in the enemy control points. They don’t reswawn until you are in the endgame world tiers. Then the enemy factions will take them back and you can free the control points and loot them again. I would spare them until 30 but would make them just before entering endgame.