Generally, the employer has a reasonable time to pay you your last check, usually within 30 days. The most common requirement is that you be paid by the next payday when you would have been paid. Some states may require that the employer pay you within a shorter or more specific period of time, either immediately or within a few days of discharge. There can be different requirements depending on whether you were fired or you quit voluntarily: some states require terminated employees to be paid immediately, while those who resign must wait until the next payday.
For more information on state final pay requirements, please see our page on Resources: State Government Agencies. At our site's listing of state government agencies, you can find the contact information and web links for the agency in your state which oversees state wage and hour laws, including final pay requirements.