There is no set rule or standard across industries for how many warnings a person receives before termination. However, individual employers may have policies such as a "three-strikes rule" that outline a standard number. From a legal standpoint, the employer merely needs to communicate policies clearly and enforce them consistently.
Employers often draw a line between automatic termination occurrences and poor performance. Even if a supervisor plans to implement automatic termination for stealing or violence, it is best to suspend the employee and investigate. Companies often ask supervisors to document more basic violations, such as tardiness or insubordination, at least a few times before termination. Documentation provides the company defense in a wrongful termination or discrimination lawsuit.