In a previous thread I talked about how my wife and I adopted a rooster - https://www.backyardchickens.com/t/826221/anyone-rescue-a-rooster-from-a-bad-home We skipped a quarantine period, and we are now at 8 days with no sign is illness. How many more days until we will be sure the flock and the new rooster are disease free?
It depends on the disease. Some diseases take only a few hours, while others won't appear for a couple weeks. By the time you've had him for three weeks or so, you can be reasonably sure that he doesn't have a disease. However, some diseases, like Chronic Respiratory Disease, are not always apparent. The bird remains a carrier, only showing symptoms during times of stress.