I have the complete opposite opinion to those two posted above and so does my instructor (who is quite internationally known and respected). You put the hours in during training. If you're training under a decent qualified master, they know you're ready for way before the black belt test recommendation. Then on the day, if the examiner knows their stuff, how long should it take for them to assess the candidates' skill?
Should it really take a day for a high dan examiner watching a candidate to say "yes, they're now a competent beginner, ready for a 1st level black belt of 9 possible levels (that will take another 40 years of training to achieve), who we don't even trust enough to let them promote someone else from white belt". It's the bottom rung on a LONG journey...
My grading syllabus is public and I have no problem saying, yes, that's really all we require - three poomsae, two free sparring rounds, one step sparring, self defence and two board breaks. Even that is more than required by the Kukkiwon. On the Kukkiwon Poom/Dan Examiner Course in Austria, 2015 they told us the testing requirements and said you can add your own in (which I've done), but I still don't need to watch a candidate (particularly one that has trained under me) for a whole day to confirm they're ready.
I think I have a more Korean mindset on this topic than a western one, but not quite as far as the Koreans take it ;-)
Anyway, just my opinion.