I was 100% told 2 years by the officer who gave me my notice , never really bothered reading it as at that point it was very much a case of 'right, no more cruises' so it may well have been 12 months with the officer not being 100% sure
you will find it in section 59 of the Police Reform Act 2002. To save you looking it up - its 12 months. Bear in mind that there is no minimum interval between warnings, so you could get a warning and have the car seized two minutes later if you don't stop doing whatever it was that was annoying the officer.