Eh, it lasted like a few seconds usually and really only happened when I was going up or down stairs or standing up suddently (I was a runner and did other sports and never had it happen, so it wasn't an exertion thing). I was just a kid and was afraid it meant something was really wrong, so I never told anyone figuring that if I did, I'd have to have tests done, they'd find out it was something serious, and that'd make it real. If no one knew and I wasn't tested, it couldn't be real. That's how kids think you know.
But yeah, it'd start at the edges of my vision turning black and staticky, and then the black edges would expand and the parts I could see would contract until they were partially or fully gone and there'd be black with bits of white static and then a 5-15 or so seconds later it'd go away. 15 seconds isn't a long time, but each time you're not sure if it's ever going to come back and it gets kind of scary. Especially if you're trying to be calm and hide it so no one finds out that there's something wrong with you.