Typically, annually or when there is a change that may impact the cleaning of the equipment.
CIP validation is a bit unique and in my eyes easier to do. For CIP you need to verify the metrics monitored are being achieved by the system for that piece of equipment. For example, if you have a tank that requires a certain minimum flow and pressure to wash you verify the flow and pressure is achieved and maintained throughout the wash. For chemicals you can utilize the literature from your chemical supplier such as their recommended time, temperature, and concentrations. Then you just verify against your CIP circuit for a wash. As an additional step you can validate with visual inspection and swabs of any type (micro, ATP, allergen if that applies). I would recommend if you do swabbing you take swabs before and after CIP of the same spots on the equipment.
An alternative to swabbing is capturing rinse water. This is better for equipment like lines or heat exchangers where visual inspection and swabbing product contact surfaces are more challenging.
Of course with heat exchangers (Plate and Frame) I would open and inspect those at least annually after CIP. May need to be more frequent depending on the products you are running and pasteurization temperatures.