If your dojo has training time requirements, then, when you get close to the number of classes specified, ask your teacher if you will be ready when you get to the required number of days. He will probably say yes and advise you to pick an uke, and if no, then he should tell you what you specifically need to work on to become ready.
While every dojo has their own little procedure for this, few teachers really want to keep track of when 5th kyu tests need to happen. The "let the teach tell you" traditions mainly come into effect for much higher ranks, or in other arts. Some of the teachers in other arts need some protection from juveniles whining to get promoted early, but this doesn't seem to be a big aikido problem.
If you have any questions about your own dojo's approach, ask any friendly senior student. They should be happy to help.