Most cases of flash blindness last for a few minutes, few hours to a day or two at most.
The length of time the flash blindness lasts depends on what caused the flash blindness.
If you've been flash blinded by a camera flash then the flash blindness from the camera flash typically lasts only a few minutes.
If you've been flashed blinded by an arc welder or other welder then the flash blindness is also usually temporary and lasts for a few hours or a few days.
I've been flashed blinded by an arc welder before when for some reason the lens fell out of my helmet in the middle of welding.
I was seeing yellow for a day and my eyes burned really bad.
I thought I was gonna be permanently blind but a few days later the burning of the eyes stopped and my eyesight came back.
Some cases of flash blindness can be permanent but most cases of flash blindness go away on their own within either a few minutes to a few hours or a day or so.