I did some reading up on it and it said that you loose your vision when trying to hold hits in to long and it cuts off oxygen from your brain and you can loose vision anywere from 30 seconds to 10 minutes.

google- blind from weed
Thehulkster321 Reviewed by Thehulkster321 on . anyone ever gone blind? hey, so i went blind last weekend when i was smoking.. it was about for 15 minutes or so. i was high.. but not f*cked up.. so anyways it was scary, and i was just wondering if anyone else has experienced that... oh and i wasn't coughing or anything.. i just slowly lost my vision until it was gone completely for about 15 min. and then i layed down for awhile and it slowly came back... soo yeah any input would be cool. Rating: 5