Under 49 CFR 180.407, what is the standard interval for an internal visual inspection of a DOT cargo tank?

49 CFR 180.407(c)

The internal visual inspection checks for corrosion, cracking, or lining damage that can't be seen from outside the tank, which is why it happens less often than the external check but still on a defined federal schedule. Knowing this baseline is necessary before assessing whether an exception applies.