Imagine, you're washing your hands after a poop, suddenly you're able to see yourself in the mirror, it only lasts 30 seconds before you go blind again, but for the rest of your life you know... You're one ugly motherfucker.
Have a timer count down in their vision. Like 100 hrs. They close their eyes and the timer pauses. They have vision for 100 hours, that's it. Choose how to use it.