If you are FO you can have it straight away, if you are lucky. Also any other base, actually. I have no clear statistics, but a course mate or mine joined MXP straight away as FO and got a transfer to AMS in about one year, for example.
You can bid for a specific base for the upgrade once you have a certain amount of hours and time in the company, and when your name is on top of the list for that specific base, you go for the command, and if you pass it, you end up there. Or like most you can upgrade in LGW as soon as you have the requirements, and then wait to transfer as a Cpt wherever you want, also according to the waiting lists.