The Cisco system should not enter standby during a call or screen share. Only when not in a call or screenshare should the system enter standby mode. Det default time is 10 minutes, can be set up to 480 minutes - webgui->configuration->standby. Exemption to this is if there is a thirdparty control system like Crestron, AMX - then the programmer of that system could have implemented a custom procedure.
otherwise could you elaborate on the problem, model, software version and so on?