The average of party nominated candidates is 9 years.
Ross Perot did not disclose, nor is there an actual requirement to.
John Kerry disclosed 7 years of his own tax records but did not disclose any of his wealthy wife's tax returns.
Romney has released one and under pressure will release two years, far below the norm for presidential candidates.
But again, disclosure is not required under the law. It is a courtesy most candidates regularly abide by.