I'm not experienced with RedGate's monitoring tools, but SSMS has an Availability Group Dashboard 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.
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:
You can find more information on querying the DMVs for this information in SQLPerformance's Monitoring Availability Group Replica Synchronization. This is the query from the article (with one additional calculated column to get you the Estimated Recovery Time):
ag.name AS ag_name,
drs.redo_queue_size / see.redo_rate AS EstimatedRecoveryTime -- Additional helpful calculated column
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
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.