That's a matter of debate, actually. Although I am not a lawyer, I have always been skeptical of lawyers who claim you should file it before 6 months. In my (non-expert, but considerably educated layman) opinion, in the event the government decided to push back on a suit that was filed before 6 months, it is likely to me that the court would rule in their favour. And really, then the only one who would have won is the lawyer that took your money; you would be no closer (and also poorer), and the taxpayers also lose the costs of defending it in court.