How reliable is the Airbus FAC calculated gross-weight?
so after having done some more research this is what I have found.The A320 has two independent gross weight calculations. When the aircraft is on the ground the FMS does the GW calculation. Prior to engine start, the current GW and CG are not defined.; predictions are computed based on the pilot entered ZFW , ZFWCG, blockfuel and the FM calculates the TOW. The GW is calculated  by adding FOB to the pilot entered ZFW (GW=ZFW+FOB)  However I don't know what this iteration rate would be.
When the A320 is airborne, the FACs independently determine the aircraft gross weight (GW) by using the aircraft angle of attack sensors. If a significant difference occurs between the FAC and FMS gross weighs (delta > 7 tons) , the  message "CHECK GW" will be displayed on the scratchpad.