I think that's the point, that prorogation isn't based on an actual written law but is more based on precedent as is a lot of the British constitution. As far as I'm aware theres just a procedure that should be followed for prorogation rather than a defined point when and for how long. In the past it has been used to end parliamentary sessions for a short period to prepare for a new one. This time however it appears that the prorogation is not being used for this purpose.
The English Court said they weren't in a position to judge on it, but the scottish court, under a different legal system, thought it was.
I expect it will be thrown out by the Supreme Court, though.