Hi, As per documentation:
Data retention | Camunda 8 Docs
Orchestration Cluster: 30 days
Optimize: 180 days
Zeebe: 7 days
Usage metrics: 730 days (2 years)
What does Zeebe retention period of 7 days mean?
A long running process which is waiting for a message event can wait a maximum of 7 days? If not, then what is the significance of Zeebe retention?
The Zeebe 7-day retention only applies to completed process instances (and their exported records), not active/long-running ones waiting on events like messages. I found the following relevant resources:
Does this help? If not, can anyone from the community jump in? 
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yes. for long-running processes check cluster size is good enough