Child abuse can come in the forms of - emotional, physical, sexual & neglect...
Whats the limit?
You need to make a decision in your mind as if the child would be better of out of the house or not (assuming the abuse is happening at home)
When to report?
You report as soon as you are sure this is happening. Also this links to the first question
Repot to who?
You can report it to social services. Im not sure but maybe the school teacher/princible might be good, if you are not sure it is defo hapeing. As these people work directly with the child and keep an eye out for behaviors from the child. But the social is the people who will go directly to the home, so you would want to be sure first of all.
How long can one suffer?
When you ask "how long can one suffer"? Im not sure what you mean, if you can explain what you mean more i will reply more?