CAA Standards Doc 11 - PPL exam records to be kept for 42 months; this is the longest an examination paper could be valid for.
JAR-FCL states records at an FTO must be kept for 5 years but does not specify for a Registered Facility.
CAA used to say 3 years but since the exam validity has been extended, 42 months looks like the lower limit with 5 years as the upper limit.