There is no limit on how long a police officer may choose to follow you before pulling you over; it is up to the officer's discretion. This is true whether you are being followed sue to speeding general suspicious behavior or even for just an expired registration tag. Further, driving with expired tags marks you for extra attention from the police. While they can stop you if your tags are expired and issue a citation, it also opens you to more potentially problematic concerns. The officer who pulls you over over can issue additional ticketss based on his observations, which may include tickets for inadequate proof of insurance and driver's license violations, etc.