I'm not experienced with RedGate's monitoring tools, but **SSMS** has an [**Availability Group Dashboard**](https://learn.microsoft.com/en-us/sql/database-engine/availability-groups/windows/use-the-always-on-dashboard-sql-server-management-studio?view=sql-server-ver15) out-of-the-box that I've always found particularly helpful with basic metrics like this. Specifically the metric in the dashboard that would be helpful to you is called the **Estimated Recovery Time (seconds)**:

> Indicates the time in seconds it takes to redo the catch-up time. The catch-up time is the time it will take for the secondary replica to catch up with the primary replica. This value is hidden by default.

I also found the **Redo Queue Size (KB)**:

> Indicates the number of log records in the log files of the secondary replica that have not yet been redone. This value is hidden by default.

And the **Redo Rate (KB/sec)** particularly helpful in monitoring the health of my secondary replicas:

> Indicates the rate in KB per second at which the log records are being redone. This value is hidden by default.

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If you want to access this information more natively so you can collect the metrics over time, the above **Availability Group Dashboard** just leverages the following **DMVs**:

1. [sys.dm_hadr_database_replica_states](https://learn.microsoft.com/en-us/sql/relational-databases/system-dynamic-management-views/sys-dm-hadr-database-replica-states-transact-sql?view=sql-server-ver15)
2. [sys.availability_databases_cluster](https://learn.microsoft.com/en-us/sql/relational-databases/system-catalog-views/sys-availability-databases-cluster-transact-sql?view=sql-server-ver15)
3. [sys.availability_groups](https://learn.microsoft.com/en-us/sql/relational-databases/system-catalog-views/sys-availability-groups-transact-sql?view=sql-server-ver15)
4. [sys.availability_replicas](https://learn.microsoft.com/en-us/sql/relational-databases/system-catalog-views/sys-availability-replicas-transact-sql?view=sql-server-ver15)

You can find more information on querying the **DMVs** for this information in [SQLPerformance's Monitoring Availability Group Replica Synchronization](https://sqlperformance.com/2015/08/monitoring/availability-group-replica-sync). This is the query from the article (with one additional calculated column to get you the **Estimated Recovery Time**):

    SELECT 
    	ar.replica_server_name, 
    	adc.database_name, 
    	ag.name AS ag_name, 
    	drs.is_local, 
    	drs.is_primary_replica, 
    	drs.synchronization_state_desc, 
    	drs.is_commit_participant, 
    	drs.synchronization_health_desc, 
    	drs.recovery_lsn, 
    	drs.truncation_lsn, 
    	drs.last_sent_lsn, 
    	drs.last_sent_time, 
    	drs.last_received_lsn, 
    	drs.last_received_time, 
    	drs.last_hardened_lsn, 
    	drs.last_hardened_time, 
    	drs.last_redone_lsn, 
    	drs.last_redone_time, 
    	drs.log_send_queue_size, 
    	drs.log_send_rate, 
    	drs.redo_queue_size, 
    	drs.redo_rate, 
        drs.redo_queue_size / see.redo_rate AS EstimatedRecoveryTime -- Additional helpful calculated column
    	drs.filestream_send_rate, 
    	drs.end_of_log_lsn, 
    	drs.last_commit_lsn, 
    	drs.last_commit_time
    FROM sys.dm_hadr_database_replica_states AS drs
    INNER JOIN sys.availability_databases_cluster AS adc 
    	ON drs.group_id = adc.group_id AND 
    	drs.group_database_id = adc.group_database_id
    INNER JOIN sys.availability_groups AS ag
    	ON ag.group_id = drs.group_id
    INNER JOIN sys.availability_replicas AS ar 
    	ON drs.group_id = ar.group_id AND 
    	drs.replica_id = ar.replica_id
    ORDER BY 
    	ag.name, 
    	ar.replica_server_name, 
    	adc.database_name;

You can create a SQL job to routinely log this information to a table so you have a historical comparison to achieve your goals with. (This is essentially what RedGate's Performance Monitor is likely doing under the hood.)

The above article also briefly mentions a third way to monitor these metrics via **Perfmon Counters**.

And finally a third and equally effective way to log your Availability Groups health metrics is via the built-in [Extended Events](https://www.mssqltips.com/sqlservertip/3524/using-extended-events-to-monitor-sql-server-availability-groups/).