Flashes in the eye occur when the vitreous fluid pulls on the retina - the light is the stimulation caused by the pulling. It can last for years and years or it can last one day. It's always best to get a dilated eye exam to make sure you have no tears beginning in the retina. I've been having flashes and balls of light and everything else for over 35 years. Never had a tear or hole, but "they" always say to have it checked. It's no tumor, it's very common - are you nearsighted? It's extremely common in nearsighted people and the more nearsighted you are the more you will get these things. I started at 19, my daughter started with that stuff at about 16.
It can look like those white flashes you see on a transmitter tower (I'm sure you've seen those when going down the road), it can look like a round ball of yellow light, it can look like a flash of cobalt blue, there are many ways for them to present themselves (and believe me, I've had 'em all!)