Hi Matt,
In my opinion Condition Length has nothing to do with Ready Time calculation it's just the custom value which you personally prefer for Alarm to be triggered.
For monitoring Ready Time higher than 3% set Condition Warning or Alert to 600ms for VM with 1 vCPU or 1200ms for 2 vCPU ...etc.
When Condition Length is set to 5 minutes you will compare data for ReadyTime from 15 samples ....
Condition Length set to 30 seconds is in this scenario useless use 20s or 40s or 1 minute ....etc.
Regards,
P.