Visa Extension Beyond One Year at a Time

I am from the USA and currently hold the visa status of "Married to a Japanese National" I've just been given my third extention for an additional year. My wife and I were hoping I would get a three year extension as she is tired of doing the paperwork, etc. every year. From your experience how long does it take to finally get a three year extension and is this simply dependent upon how long you've lived in Japan, or are there other factors involved such as employment status, etc.?

Usually you can get a three year extension the first or second time you extend your visa, but that may depend on your household's declared revenues in Japan.