The rule of thumb is for a fault the secondary current should not exceed 20 times the rating of the CT. In the U.S. CTs are typically rated for 5 A secondary, so this would typically be 100 A. I think this rule of thumb follows from the ANSI standard for CTs (which I believe is C57.13), so that you do not exceed the knee point of the CT's excitation curve and risk saturating the CT. It doesn't mean you can never exceed 100 A, but you need to be careful when you do.
To be clear, this is only for fault current, which will only last a short period of time. For continuous load, you usually do not want to exceed the secondary rating of the CT, i.e. 5 A. However, most CTs have a built in termal overload factor, often 2 to 3 times the secondary rating--for emergency loading.