As one who used to conduct a lot of CFI checkrides, maybe I can explain why some orals take so long. Number 1, the CFI is the most important certificate issued by the FAA. A marginal CFI can create a chain of circumstances that results in negative effects for decades. Most examiners and inspectors will go to great lengths to ensure the applicant knows what he or she is doing. Number 2, the examiner must ask questions from each area of the PTS. You're not allowed to ask a few questions in a couple of areas and assume knowledge there will transfer to all other areas. You'd be amazed to hear the myths and misconceptions that some otherwise knowledgable applicants have accumulated. Number 3, and possibly the most pertinent, most examiners don't want to fail applicants. Therefore, when a weak applicant shows up, the oral exam becomes a teaching session that lasts until the examiner is comfortable with the applicant's knowledge level. Sure, this isn't the way it's supposed to work and the FAA seriously frowns on this. But, some examiners will routinely revert to being a teacher in order to avoid issuing a pink slip. I doubt that a prepared applicant will ever receive an 8 hour oral.