MS calculates the period of time it shall switch on its receiver based on a few parameters like: BS_PA_MFRMS – defines the number of 51-multiframes for periodicity of PCH subchannelnpaging decoding and it may range from 2 to 9.
BS_CC_Chans, CCCH_GROUP, Paging Group. IMSI is also used for that calculation.
As far as I remember they are transmitted on SI3 (BCCH).