Players can earn Double XP codes, Double Weapon XP codes, and Double Division XP codes from Supply Drops in Call of Duty: WWII.
These items are considered Bonuses, and the times for how long they last vary by which one you are awarded in a Supply Drop. The longer the time, the higher the rarity of the code.
As seen in the image above, we opened a Rare Supply Drop in Call of Duty: WWII and was awarded 10 minutes of Double Weapon XP and 5 Minutes of Double Division XP.
You can also keep track of how many boosts you have active in the Solider tab in the game menu. Go to Active Boosts to see what you currently have live, and it shows you for how long it will be live for.