In a scenario where an actual link down occurs, failover should occur immediately upon detection of the link down. If the link is not going down (i.e. due to some media converter in between) the LACP timeout timer will be the determining factor in link failover.
The default timeout period for LACP in EXOS is 90 seconds. LACP timeout can be configured as either long (90 seconds) or short (3 seconds). If the default timeout is too long, it can be reduced by typing the command "configure sharing <port> lacp timeout short". The down link will now be removed from the aggregator after 3 seconds.