There is no official documentation for the max. latency supported between FMCs. Unfortunetly it is not documented what kind of replication FMC is using (async vs sync), the only thing I know is that there is some sybase and file replication going on between the devices... In case the database (sybase) replication was synchronous I would not recommend > 20ms latency between FMC but I guess you will need to reach out to Cisco for a proper answer.
