I tried to read CFX user guide about stage and frozen rotor interfaces, but i cannot understand precisely in which case is better frozen rotor and when is better stage.

Can anyone help me?

Thanks a lot.

For this thumb rule is that for study state problem frozen rotor is suitable and for transiant problem u can go through stage. Stage will give u better result but problem is that you required more resources for that i.e.memory and time so better you go through frozen rotor.

We use frozen rotor analysis for full geometry simulations at steady state but use stage when using only a section of the geometry for a periodic set-up. Both approaches have their advantages and disadvantages. For detailed information,I would refer you to

ISSUES SURROUNDING MULTIPLE FRAMES OF REFERENCE MODELS FOR TURBO COMPRESSOR APPLICATIONS Zheji Liu and D. Lee Hill Dresser-Rand, Olean, NY USA

Take a look in the best practice guidelines for turbomachinery CFD in CFD-Wiki here at CFD Online. There is a section about this there. Some of the other comments you got here are not fully correct.