I attended Blizzcon in 2011 and due to some extreme partying on Friday night, I missed some of the discussion panels that I had wanted to see on Saturday morning.
. I actually purchased the Virtual Ticket that evening so I could go back and watch them later. The whole con was saved via the Virtual Ticket and I was able to go back and watch it up to a week later.
I don't believe there is a limit on the amount of Virtual Tickets they sell or a time limit on when you have to purchase them.