If the poll control cluster are used, the default poll intervals can be set in the constructor of an application class.
e.g. ZigBee Switch application : ColorSwitchApplication.cpp
If the poll control cluster are not used. The function void EnablePolling(const CCompactTimerService::ticks_diff_t interval) could be used to start or change the polling interval.
e.g.
aps.m_nwk.m_nlme.EnablePolling(CCompactTimerService::ticksPerSecond / 4); // 250ms
Would change start the ZigBee end device to poll in a 250ms interval.