How long does a teacher has to report child abuse?

The question is vague. If you are asking how long does a parent have to sue a teacher who falsely reported child abuse, in California, the answer is that you cannot sue a mandated reporter such as a teacher for making false allegations of child abuse to the police or a child protective agency. This is so even if the teacher or other mandated reporter knowingly made the false report.

The rationale is that those who are required by law to report suspected child abuse or neglect should be protected from not only from civil liability for making false reports, but from being sued at all. Were it otherwise, mandated reporters would be reluctant to make any child abuse reports at all and might look the other way for fear of being sued. Even if a mandated reporter simply made an error in reporting suspected child abuse, a parent could cause him or her great misery, expense and potential civil liability by suing them, claiming that the reporter knowingly made the false report. To prevent that possibility and encourage mandated reporter to report suspected child abuse, you cannot sue them; they have absolute immunity.

Non-mandated reporters stand in different shoes in that you may sue them for knowingly making false reports of child abuse. Hence, they have only a qualified immunity from civil liability where they only negligently/unintentionally make false reports to the police or a Child Protective Agency.