The thing is, let's say you know one, but she's always an ass. Then she's not a tsundere, just a bitch. On the other hand, if she gradually opens up and admits her feelings, she's tsundere, but for how long? If she admits she loves you, is she still a tsundere?
So I think tsundere is a stage, the process of going from bitch to a maybe somewhat shy but mostly normal girl. If there's no change over time, than she's not tsundere, but if there is change over time, eventually that change comes to an end, and she's no longer tsundere.