I am just doing a mathematical calculation. One LTE radio frame is 10 ms long and one radio frame has 10 sub frames, 1ms long each. One sub frame consists of two slots and each slot is 0.5ms long. In case of normal cyclic prefix, one time slot has 7 OFDM symbol, so that length of one OFDM symbol is 0.5 ms / 7 = (0.07142) ms long. PSS and SSS both are transmitted in last and second last OFDM symbols of radio sub frame '0' and '5'. In this way, we can say total of 2 OFDM symbols ( 2* 0.07142ms) time is necessary to read PSS and SSS. Once PSS and SSS are read by UE, UE known CGI.