Actually as the others have mentioned, grading varies from one dojo to the next. In our dojo you need to pass a certain amount of lessons before you go for a test, but it's truer for the first kyu's and less for higher kyu's. Usually it's at least 40 lessons for 5th kyu, 60 more for 4th, 80 more for 3rd, 100 more for 2nd...
But I know some grade before they have that amount of classes if they feel ready and are allowed to test.
In our dojo, when you want to grade, you go to a higher ranking student (at least 2 kyu's higher than you) and ask them if they think you are ready. If they say yes, it is thier job to train you for your exam so you will know the techniques and all that is expected from you. Usually they will also be your uke in the exam.
I went to my first test after 6 monthes and the next only a year later even though my teacher told me I can grade. I just didn't feel ready and I was never in a hurry. For me, aikido is a way of life and I hope to do it as long as I live, so why rush when there is so much still to learn?
But it's really how you feel. I wanted to feel a change inside as well as knowing how to do the technique well.
We have someone in our dojo that started training in the UK and I know they used to grade around every 3 monthes or so. But I guess it also depended on the amount of days and hours you train each week.
In any case, when you decide to grade- GOOD LUCK to you!