Incense are items that lure Pokémon to your location. One activated Incense typically lasts for 30 minutes but may last longer during special events. You can also activate up to 200 Incense at a time to stack their effects instead of using one every 30 minutes. 
Note: Incense duration has been extended to 1 hour until further notice. Learn more about this change and other changes.
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Pokémon GO Game GUIDE