PRS ticket rule is as follows. (eticket WL auto canceled) so giving you PRS exact Rule for Wait list ticket. Time limit depend on the Distance of KM too.
UNUSED WAIT LISTED OR RAC TICKETS (PRS Ticket):
(1A) Subject to provisions of subrule (2), no cancellation charges shall be payable if a wait-listed or RAC ticket is presented for cancellation up to :
(i)...
more... Three hours, when the ticket is for a destination station up to 200 KM.
(ii) Six hours, when the ticket is for a destination station of more than 200 KM but up to 500 KM., and
(iii) Twelve hours , when the ticket is for a destination station of more than 500 KM.,
after the actual departure of the train except for a deduction of a clerkage charge of Rs. 20/- per passenger.
Provided that for night train leaving between 21:00 hours and 06:00 hours (actual departures), refunds shall be admissible at the station within the time limit specified above or within four hours of opening of the reservation office, whichever is later.