In my first association, we had two gradings in eight years. My current one is more along Peter & Wynands'. Gradings are held every quarter, there is a minimum of 6 months between a persons kyu gradings (unless previous experience warrants faster grading) and a year between 1kyu and dan grade. Time between dan grades varies, but there's a 2 year minimum between 1st and 2nd dan.
Wynand, go for it, as long as you're enjoying it and your ukes can cope with your enthusiasm for grading