It depends really because it has just changed that from Senior to CSS it's all done on performace monitoring and then you are invited for an interview. This changed as of 2 days ago!!!
It's the worst promotion system in the world! When you start as a junior you can move to senior from anything from 6 months to 5 years, it all depends how much you are noticed by the CSS!!! When you get your seniors you used to be able to apply for CSS after 6 month, but now this has changed I gueses this too means it could take 6 months or 6 years!!!! It is the worst promotion system in the world. It works well for the gay lads though!