Usually could be stored at -20ºC, there are recommendations of differents laboratories ensuring conservation between 4 -6 months. If you need boost security, You must use freezing at -80 ° C.

http://www.nmslabs.com/tests/Cortisol-and-Cortisone--24-Hour-Urine/7655UH

http://www.questdiagnostics.com/testcenter/BUOrderInfo.action?tc=11280X&labCode=SJC

If you dont store your urine samples too long, -20° should be fully sufficient. Cortisol ist rather stable and no inactivating enzymes apper, to my knowledge, in urine. For longer storage times, however, lower temperature might be safer.

Thank you very much for your answer.