Do you hold a UK JAA licence? If so LASORS states that the initial LST has a six month validity period and that the minimum of six landings must be conducted within this period AS WELL AS the type rating application form being submitted to the CAA.
LASORS F4.2 refers. Note I have an old 2007 edition so you might want to get the latest version and if you don't hold a UK licence then you would need to check with your local CAA to see if they have the same interpretation.