Oh, you just have to know exactly when you're going on sale. A lot of the time they will set up an exact time, like "9:00AM CST" or something for you to go buy them, and they can sell out anywhere from 2 minutes to 15 minutes. A lot of times there will be open seats still though, but all the good seats sell out in the first 2-3 minutes I'd say...it also depends on how big of a venue you're going too, and how many people are estimated to go to the show/buy the tickets.