Sadly it's a "How long is a piece of string" answer. I know a CSA2 that went into a CSA1 only station and got higher grade working from day 1. They then went into a CSA1 position within 5 months (at the same station).
CSAs have access to an app that allows them to choose (nominate) a different station - usually this would be to get them closer to home (who wants to work at Heathrow is they live in Upminster!)
But... Nominations can also be made to larger, usually central location, stations that always need CSA1s due to staff turnover. So CSA2s often nominate to one of these big stations to advance their CSA1 ambitions. I've not met a CSA2 that didn't become a CSA1 by 12 months, although I'm sure they exist. (Of course, it's not compulsory, and some CSA2s are happy to remain a CSA2.)
I'm not 100% sure on this, so will defer to others with better knowledge.
What I will say in summary is that "business needs" can, and does, supercede any written policy that LUL publishes - they simply bend the rules to suit their needs.
Edit: there are no silly questions.