When using Allways On with asyncron secondary server, the secondary server sometimes fall behind if lots of changes on primary server. But how far behind? I am using this command to get a picture of the status. The command is executed on primary server.
select recovery_lsn
,truncation_lsn
,last_hardened_lsn
,last_received_lsn
,last_redone_lsn
,last_redone_time
from sys.dm_hadr_database_replica_states