One mission is around 30 min. It can go on for as long as you like. There are 6 or 7 different missions, and you can do one after the other.
When you sign up you go to one of several KP stations, put your park ticket in a machine and get a return time on a ticket like a FP. There are stations in one of the Innoventions buildings, (maybe both,we always get them from the same one) at a KP station that is near that bridgeway from FW to WS, and then there are kiosks in WS at Norway and the International Gateway (again there maybe others, but I don't recall as we generally go to one of those two). The ticket will give you a return time and location. Once you get there, you'll be given a Kimmunicator and told which country to head to for your first mission. If you've already done a mission in a particular country, you can tell the CMs and they can send you elsewhere.
It is a lot fun for kids and adults. Enjoy!