How long is one's service record kept after leaving the Service?
Service records for those who left before 1920 have been transferred to the National Archives, and MOD has been in discussion with the National Archives about the transfer of service records for those who served after 1920 - particularly those who served during the Second World War.
I would expect MOD to retain service records for as long as they might reasonably need access (perhaps for 80 years from the date of enlistment) and then transfer the records to TNA.