What is the lowest salary required for salaried employee’s?

There is no legal limit for how low a salary can be. A salary is just how you pay it: e.g. in equal weekly amounts, without counting hours worked. Being paid a salary has nothing to do with the amount.
If you mean how low can it be for an employee to not be eligible for overtime (since salaried employees *can* get overtime, though the calculations are more complex than for hourly staff), the threshhold is approximately $23,600 per year.
However, it's not enough to be paid a sufficient salary: to be exempt from overtime, the employee must also meet one or more of the tests or criteria to be exempt; her job duties and authority must meet certain tests. You can find these tests on the U.S. Department of Labor website under "overtime": compare them, especially the "administrative employee" criteria, to your wife's job. If she meets at least one test, then based on how much she earns, she would not get overtime. But if she does not meet at least set of criteria, then even though she is salaried, she likely is entitled to overtime; in that case, if works more than 40 hours per week without getting overtime, contact the Dept. of Labor to file a complaint.