Just to add a little to the info usefully provided by up4it:
The fixed penalty office has to post a NIP so that in normal circumstances it will arrive at the registered keeper's address within 14 days of the offence (day 1 being the day of the offence).
However, if it does not arrive within this time a prosecution is not out of the question, but it will be up to the FPO to show the court that they took all reasonable steps to ensure the notice arrived in time. A prosecution is not automatically ruled out if, say, the notice was delayed in the post.
The absolute limit for prosecuting speeding offences is six months.