Final answer:
Proficiency demonstration and sustained certification for welding require regular welding activity documented in a Welder's Log. Failing to maintain proficiency will require recertification, but the specific number of days is not given.
Step-by-step explanation:
Proficiency demonstration and sustained certification for welding requires maintaining a Welder's Log documenting the welding of at least 1 ferrous and 1 non-ferrous item every 30 days in any process. The specific requirements can be found in NAVAIR 01-1A-34. According to NAVAIR 01-1A-34, if a welder fails to maintain proficiency for a certain number of days, they will need to recertify. Unfortunately, the exact number of days is not provided in the given information.