That's a hard question to answer because it varies so wildly from server to server...it also depends largely on what role you are. For instance, it's easier to replace DPS because there are so many more DPS than tanks or healers. This also means that you're likely to have people parse you and other DPS applying for the same position to see who fares better.

A healer or tank is less likely to be replaced.

It also depends on what the endgame raiding scene is like on your server. I've seen reports of servers where the endgame scene is basically nonexistent, and I've been on some where it's very lively and easy to find someone to play with.

As a heads-up, I would not recommend trying to find a new static if you're looking for a spot for more than just one person. Some friends and I have had this issue in the past. If you plan on going into a static with a specific person, it will likely be easier to just form your own.

Hope that helps!