There is no time limit, and they may be conducting further investigations, including (if the child is school age) interviews of the child at school, and interviews of teachers, counselors and others who are in frequent contact with the child. She should immediately retain a criminal defense attorney. Child cruelty cases are taken very seriously, and will often result in felony charges. Your mother needs to get ahead of this matter and not wait.
The depends on the specific section of the law charged. Generally, in Maryland there is no statute of limitations for felonies, and for misdemeanors it also depends on the offense charged.