Hi there, Six Flags Flash Pass allows you to come anytime after the pager has notified you it is your time to ride. I have never experienced a situation when it has timed out on me. I've let it sit for a good 45 mins or longer and was still able to walk on to the ride.
As far as Gold or Platinum, that depends on how much you want to spend. It's been several years since I've used one but last I check Platinum allowed you to re-ride a ride immediately after you got off of it as a bonus. I believe that Platinum as a lower wait time than either the Gold or Basic Flash Passes as well. I've always gotten either Basic or Gold and have been satisfied with my choice.