Replication delay

Hi,

I have a pair of PG servers, a master and a replica, all read-write queries are handled by the master, read-only ones
areby the replica.   

From time to time the replica itself is too busy, all read-only queries will get  inconsistent results because of
replicationlag,  sometimes it can be longer than 6 minutes.  I am thinking to add multiple replicas to off-load
read-onlyqueries, can you please suggest a way to monitor and failover the read-only query when the replication lag in
areplica is more than 5 minutes? 

regards