This is a component of ocular migraine
, which has its origination in the occipital lobes of the brain.
Most are of short duration, minutes to 1 hr, although the visual diminution in the center can be very disturbing. Ocular (occipital) migraine is benign, causes no permanent changes and clears with no after effects. If very frequent, you could seek the help of a neurologist but this is uncommon.