There are multiple factors. First, your income determines whether the minimum length of your case is 3 years or instead 5. Second, an otherwise 3-year plan is allowed to go as long as 5 years in order to pay certain debts that have to be paid during the course of the case. Those debts may be secured debts—a vehicle loan or to catch up on a mortgage, for example—or “priority” debts—recent income tax debt or unpaid child/spousal support payments. Third, an otherwise 3-year plan is usually allowed to go as long as 5 years simply stretch out the Chapter 13 payments to reduce them to make them affordable. Fourth, plan payments may be adjusted months or even years after filing if there are changed circumstances, resulting in a longer or shorter case. Finally, under any circumstances, cases are not allowed to take longer than 5 years.