it varies depending on the contravention and regs
eg
they have 28 days from date of contravention to send a bus lane parking pcn
london local authorities act 1996
sec II
4 (3) A participating council may not serve a penalty charge notice under this section after the expiry of the period of 28 days beginning with the date on which the penalty charge allegedly became payable.
http://www.opsi.gov.uk/acts/locact00/20000007.htmfor a moving traffic violation in a bus lane the regualations have no limit
london local authorities act 2000
part II
4.—(1) Where a council, on the basis of information provided by the use of a prescribed device, has reason to believe that a penalty charge is payable under this Part of this Act with respect to a vehicle, they may serve a penalty charge notice on the person appearing to them to be the owner of the vehicle
http://www.opsi.gov.uk/acts/locact00/20000007.htmhowver i would suggest longer than 6 months is unfair
following patas davies v kensington
so what regulations are on your PCN ?