Cleans test run results from Controllers or load generators in the following instances:
If the results have already been collated.
There are old performance tests whose results were not collated.
If the performance test was deleted.
The RESULT_EXPIRY_DAYS task parameter should be between 1 and 1000.
Default frequency: Every 6 hours
When defining the RESULT_EXPIRY_DAYS value you should take into consideration that the System resources play a very important role during the Load test.
For Temporary directory space you should ensure that the temporary directory (i.e. Environment variables -> TEMP and TMP) point to a location that has enough disk space.
So the answer to your question is that you should use best judgment which is the optimal time not collated results to be deleted on your specific environment. It will depend on tests you are running and the size of the result folders they are generating.