14 days for the notice of intended prosecution, which can be verbal if stopped on the side of the road, and if not, has to be in writing. problem is, if they claim to have sent it but you have not received it or responded to it. BUT, first thing you get by post in the Notice of intended prosecution, so if what you have received is not a NIP, it may be you were served before....? If it is, its outside the period. BUT this may also be a scam, so maybe worth cehcking with local plod!!
BTW, I am assuming this is an offence in the UK we are talking about, I've had stuff filtering through form Holland over many many months in the past, fines, for speeding when I was nto even there even!!