When you set a Recovery Point Objective (RPO) value during replication configuration, you determine the maximum data loss that you can tolerate.
The RPO value affects replication scheduling, but vSphere Replication does not adhere to a strict replication schedule. For example, when you set the RPO to 15 minutes, you instruct vSphere Replication that you can tolerate losing the data for up to 15 minutes. This does not mean that data is replicated every 15 minutes.
If you set an RPO of x minutes, and the RPO is not violated, the latest available replication instance can never reflect a state that is older than x minutes. A replication instance reflects the state of a virtual machine at the time the synchronization starts.
Assume that during replication configuration you set the RPO to 15 minutes. If the synchronization starts at 12:00 and it takes five minutes to transfer to the target site, the instance becomes available on the target site at 12:05, but it reflects the state of the virtual machine at 12:00. The next synchronization can start no later than 12:10. This replication instance is then available at 12:15 when the first replication instance that started at 12:00 expires.
If you set the RPO to 15 minutes and the replication takes 7.5 minutes to transfer an instance, vSphere Replication transfers an instance all the time. If the replication takes more than 7.5 minutes, the replication encounters periodic RPO violations. For example, if the replication starts at 12:00 and takes 10 minutes to transfer an instance, the replication finishes at 12:10. You can start another replication immediately, but it finishes at 12:20. During the time interval 12:15-12:20, an RPO violation occurs because the latest available instance started at 12:00 and is too old.
The replication scheduler tries to satisfy these constraints by overlapping replications to optimize bandwidth use and might start replications for some virtual machines earlier than expected.
To determine the replication transfer time, the replication scheduler uses the duration of the last few instances to estimate the next one.