When to shorten your board?

At what point do you turn around and start going shorter with your board.
The reason I ask is I am using a foam board at the moment and am looking into getting a hard board in the new year, what I was thinking is either getting another mini mal the same size as what I've got now in the early months of the year or wait for the summer and keep using my foam board until then and then buy a fish or something like that.
What skill set do you need to be at before you can shorten your board?
The reason I ask is I am using a foam board at the moment and am looking into getting a hard board in the new year, what I was thinking is either getting another mini mal the same size as what I've got now in the early months of the year or wait for the summer and keep using my foam board until then and then buy a fish or something like that.
What skill set do you need to be at before you can shorten your board?