It's 7 minutes per 1 energy, if you get down to 0 energy you would need to wait 15*7=105 minutes, which is about a hour and a half. Considering how there are new levels, finishing them all would take MONTHS. Especially Insane Mode Overlord's Den and other end-game levels.
In a survival situation like that I would never rely on weapons with a limited ammo. Despite their strenght and range I'm not a good shooter and I would most likely kill everything but zombies.
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