Darn good question! I think the checkride should be the length of a mini skirt, long enough to cover everything but short enough to keep it interseting. Ok, joking aside, most of my checkrides run about 4 hours from the time you walk in the door to the time you walk out. The checkride breaks down to 30 minutes of pre-checkride paperwork, 1 hour 30 minutes for the oral, 15 minutes for a quick break and preflight, 1 hour 30 minutes for the flight and another 15 minutes for debrief and paperwork. Pretty streight forward.
Those are average times. Some people take a little longer and others a little less. What makes that determination is how well the applicant is prepared. For example, I have seen a very well prepared private pilot applicant pass my oral in 1 hour and five minutes and I've had private applicants take 2 hours 10 minutes and still pass. The flying part is very consistant at 1.5 plus or minus .2 on the hobbs.
How can an I keep my checkride orals to 1 hour 30 minutes when anouther examener takes 4 hours? I use several senerios throughtout the test plus I combine Tasks from the PTS. Here's an example of how I combine Task from the PTS. Noticing the preplaned cross country passes through Class C airspace I'll ask according to the forcast, when will the XYZ (Class C airport) be VFR (IFR)? This allows me to evaluate the ability to read a TAF and the applicants knoweldge of VFR/IFR mins in Class C airspace. Note that this isn't the only question I'll ask on the subjects, it's just one example on how combine testing Tasks.
Checkrides very from applicant to applicant. Some folks are more nervious than others and some are better test takers than other. I recently had a CFI have an applicant come to me for a private pilot checkride. The applicant passed the test, but the CFI was a little P.O. that the oral took over 2 hours. The issue here was with how the applicant was prepared. Some of the questions I initially posed to the applicant he would lock up on. It took a little effort to pry the knoweldge out of him. For example, When I ask most applicants to show me that the aircraft is airworthy for the flight we are about to make, it takes less than 5 minutes to take the log books and show me the requried inspections. With this applicant when the question was posed he locked up. He grabed the logs and started paging threw them with a glassy eyed look. I let him flounder for a bit and then ask him what he's looking for and he list the required inspections. A good sign. Now I ask where those inspection could be found. He continued to work through his list he made with the logs and he found everythings. It took a little more than 15 minutes to do that. But as you can see, if we have to do that for 3 or 4 questions, the oral can get quite long. (Hint: have your student put together a cheat sheet with the required inspections and tag them with sticky notes in the logs).