You can use a Timer for that: https://docs.unrealengine.com/latest/INT/Gameplay/HowTo/UseTimers/Blueprints/
Basically, when the player gets the power up mesh you set the Timer (5 seconds). When the timer has elapsed, it calls either a Custom Event or a Function. In it, you can remove the power up. If the player gets another power up in the meantime, the timer is set again, which means it starts counting again from 5.