A permanent installation boat ladder should be inspected every six months to a year for deterioration or problems with installation.
A boat that is in the water and always on the boat will only need an annual inspection but it won't have to worry about corrosion because it's not constantly exposed to salt water like a boat that spends time docked.
If you're using a swim step ladder as an emergency exit then inspect monthly before use, if used often then inspections can happen quarterly and if infrequently then they'll need yearly inspections instead.