How long will sundowning continue after open heart surgery?

Highly variable : But usually longer with advanced age and prolonged ICU time.

It varies: If indeed it is sundowning, and not something else (medications, hypoxias, infections, metabolic, etc), it should improve within days as the brain recovers and a more normal routine is instituted. Sometimes, recovery is not complete until the patient is home amidst more familiar surroundings.