I'd say twice a year is regular intervals, btw.
my point was that you aren't expected to take a test twice a year. It is just that the option is available. For example, I waited more than 4 years between testing for 1kyu and shodan. I didn't attempt it and fail it 8 times, I simply waited until I was ready. For me, that took a while due to a number of factors including health and free time etc. At school though, it is expected that you test at regular intervals.