In the US, they recommend once per year.
In Europe, they recommend once every two and a half years (ie. half the time between hyrostatic tests).
My personal view (as a former tank inspector for a dive shop) is that unless you have reason to believe the tank is contaminated, visual inspections are a complete and utter waste of time.
Daily
before use
For fish tanks, weekly, at a minimum.
Septic tank drains should be inspected and cleaned every 3-5 years to ensure they are functioning properly.
The whole vehicle should have a safety inspection every service.
Fire extinguishers should be inspected annually - by a suitably qualified person.
Radiator hoses and heater hoses should be inspected yearly and replaced when neccessary.
The error rate is less taht 1%. You should have your valves inspected often.
The contents of any emergency kit should be routinely inspected, depending on the anticipated necessity and/or the current disaster "season." Depending on where you are, it should be inspected and replenished every 6-15 months, if not more frequently.
The purpose of a septic cleanout cap is to provide access to the septic system for inspection, maintenance, and pumping. It should be inspected and maintained at least once a year to ensure proper functioning and prevent potential issues.
They are inspected at least once a year. Inspections are random - and without warning.
Vertical flow hoods should typically be inspected at least once a month to ensure proper functioning and maintain sterility. Additionally, they should undergo a thorough certification or performance validation annually, often conducted by a qualified technician. Regular maintenance, such as filter replacement and cleaning, should also be performed according to manufacturer guidelines or when visual inspections indicate a need.