If your employer was willfully paying you less because of your sex, race, or ethnicity, you have three years to file a claim since the violation occurred. But if your employer's actions were not willful, then you have two years to file. Each paycheck you receive that pays you less than you are entitled to counts as a "violation."
Example: Tom purposely pays his employee Patty less than her colleagues because she is female. After five years of receiving paychecks that are too low, she finally files a claim with the Labor Commissioner. Due to the three-year statute of limitations, the Labor Commissioner can remedy only the last three years of underpaid wages.
Note that employers are required to maintain your wage record for at least three years. This way, all the evidence of your wages will be available should you bring a wage claim.