Federal law under the FLSA requires payment for all time worked, but it does not mandate a deadline for the final paycheck. That is left to the states, and they differ substantially.
The deadlines
Same-day or near-immediate payment for terminated employees is required in states including California, Colorado, Massachusetts, Missouri, Montana and Utah.
Most other states allow until the next regular payday.
Four states — Alabama, Florida, Georgia and Mississippi — have no state-specific final paycheck law at all, so the federal default applies.
Quitting versus being fired
The separation type affects the deadline in most states. Terminated employees typically have shorter final pay windows than those who resign.
Missouri has a final paycheck law if you are fired, but not if you quit.
In California, an employee who quits without notice must be paid within 72 hours; one who gives at least 72 hours' notice must be paid at the time of quitting.
What must be included
All wages earned through your last day, prorated for the partial pay period. For hourly employees that means hours at the correct rate, including any overtime triggered in that partial period.
Earned commissions and bonuses generally must be included once the amount can be calculated.