Back in the UK, we didn`t tend to focus on gradings much, I only graded 4 times in 9 1/2 years of regular training. Gradings tended to be done in the early days when Sensei felt like it, or felt that someone was ready. After a health scare, he took a sabbatical, leaving the dojo in the hands of the Sempai. During this time there were no gradings, the sabbatical became permanent, consequently gradings ceased pretty much altogether. The situation changed after the the sempai became adopted by a couple of Sensei from other clubs. Now gradings are held regularly again, usually by recommendation.
Our Aikikai gradings here in Japan are based on the number of sessions trained. There are a minimum number of sessions required for each kyu and Shodan, a record of attendance is kept, once these have been reached or surpassed, one of the Sensei will let you know and give you the form to fill in to apply for the next test. Tests are held every 3 or 4 months.