A coil
 

How often do ya clean an A COIL on home A/C unit?

Not very often if at all. The duct filter should keep it clean. They are difficult to clean because of all the water that would drain back into the furance along with the complication of getting access to it.

Tanks !! But when do ya know if it needs to be cleaned?

When performance is lacking and/or pressure checks are not withing acceptable range and all other causes have been checked.
You may have a panel that can be removed to do a visual inspection.