Holly Faulkner, Project Assistant says:

Hi Kim, Male bluebirds do not typically incubate eggs, however we recommend waiting at least 4 weeks from the last time you saw an adult before assuming the nest is abandoned. It is against federal law to disturb or remove an active birds nest, so it is best to be on the safe side here – birds can be sneaky. Once that period has passed, and you’re sure there is no breeding activity, the nest can be considered inactive and you can clean the box out if you wish. Please send any further questions to NestWatch at nestwatch@cornell.edu – a citizen science project here at the Cornell Lab that deals with nesting birds. These comment sections are not monitored regularly.