Its impossible to know without any information (power through the LED array, efficiency, desired junction temperature, thermal resistances between all interfaces, heatsink rating, ambient rating, airflow, etc).
The critical value is the junction temperature of the dies - which is impossible to measure. If you have the various junction resistances, you can measure the outer package thermal to determine if there is enough cooling present. Or you can be very conservative and simply make sure its not "too hot" (<< 50C for aggressive cooling) in your ambient environment.