I have been in several aikido dojos, as well as a karate dojo, and they all had different shodan requirements and testing times. In brief, with all of them, the sensei told the the students that it was time for them to prepare (or take!) the test. One aikido dojo had very long tests for shodan 1-2 hours. In another they ran about 30 minutes. In another dojo, sometimes the sensei would stop a regular class in the middle and say "you! Sit in the middle of the mat!" and would run a 15-20 minute test, and that was it! The karate dojo (kyokushin) had about a ninety minute test - - the last 30 minutes were ten consecutive 3 minute sparring bouts with fresh opponents.
IMHO, I think calling these things 'tests' is really inappropriate. I would prefer to call them 'recitals'. It's really a performance (showing what you know) rather than testing (what you don't know). I don't think you should be testing for a Dan rank until the head of the school tells you to prepare for it.