After some digging it looks like there are two parts to this and the time limits are set in Law:
First:
The Time Limit for telling the Police (etc.)
who was driving is stipulated in the Road Traffic Act 1988 (Note - AFAIK Scotland is different):
Section 172:
Duty to give information as to identity of driver etc in certain circumstances.
(1)
This section applies—
(a) to any offence under the preceding provisions of this Act except—
(i) an offence under Part V, or
(ii) an offence under section 13, 16, 51(2), 61(4), 67(9), 68(4), 96 or 120,
and to an offence under section 178 of this Act,
(b) to any offence under sections 25, 26 or 27 of the Road Traffic Offenders Act 1988,
(c) to any offence against any other enactment relating to the use of vehicles on roads, and
(d) to manslaughter, or in Scotland culpable homicide, by the driver of a motor vehicle.
(2)
Where the driver of a vehicle is alleged to be guilty of an offence to which this section applies—
(a) the person keeping the vehicle shall give such information as to the identity of the driver as he may be required to give by or on behalf of a chief officer of police or the Chief Constable of the British Transport Police Force, and
(b) any other person shall if required as stated above give any information which it is in his power to give and may lead to identification of the driver.
(3) Subject to the following provisions, a person who fails to comply with a requirement under subsection (2) above shall be guilty of an offence.
(4) A person shall not be guilty of an offence by virtue of paragraph (a) of subsection (2) above if he shows that he did not know and could not with reasonable diligence have ascertained who the driver of the vehicle was.
..........
(7)
A requirement under subsection (2) may be made by written notice served by post; and where it is so made—
(a) it shall have effect as a requirement to give the information
within the period of 28 days (my highlighting) beginning with the day on which the notice is served, and
(b) the person on whom the notice is served shall not be guilty of an offence under this section if he shows either that he gave the information as soon as reasonably practicable after the end of that period or that it has not been reasonably practicable for him to give it........
(10)
In this section—
“registered address”, in relation to the registered keeper of a vehicle, means the address recorded in the record kept under the Vehicles Excise and Registration Act 1994 with respect to that vehicle as being that person’s address, and
“registered keeper”, in relation to a vehicle, means the person in whose name the vehicle is registered under that Act;
and references to the driver of a vehicle include references to the rider of a cycle.
You're looking at a £1000 fine for failing to provide the info (
https://www.legislation.gov.uk/ukpga/1988/53/schedule/2 and
https://www.legislation.gov.uk/ukpga...on/122/enacted and possibly court costs, prosecution costs, a surcharge and 6 points
.
AND
The Time Limit for the summons is stipulated in the Road Traffic Offenders Act 1988:
Section 1
Subject to section 2 of this Act, [F1a person shall not be convicted of an offence to which this section applies unless]—
(a) he was warned at the time the offence was committed that the question of prosecuting him for some one or other of the offences to which this section applies would be taken into consideration, or
(b) within fourteen days of the commission of the offence a
summons (my highlighting) (or, in Scotland, a complaint) for the offence was served on him, or
(c)
within fourteen days (my highlighting) of the commission of the offence a notice of the intended prosecution specifying the nature of the alleged offence and the time and place where it is alleged to have been committed, was—
(i) in the case of an offence under section 28 or 29 of the Road Traffic Act 1988 (cycling offences), served on him,
(ii) in the case of any other offence, served on him or on the person, if any, registered as the keeper of the vehicle at the time of the commission of the offence.
Hope that helps.