By doing the upgrade on a cloned instance would give you approximate time. Based on your test env you can estimate how much its going to take on production.
I see you are already on R12.2.4 and R12.2.9 is an online patch, then why are you so much concerned about timing?
- Q: Is there a direct upgrade path from EBS 12.2.x to 12.2.9?
- A: Yes. Release 12.2.x customers can apply 12.2.9 directly to their environments. EBS 12.2.9 is an online patch, so it can be applied while an existing Release 12.2.x system is running.
Like yoonas said, it's better to test in non-prod first.
I think most time will be spent on preparation: you need to prepare list of patches and EBS components to be upgraded.
Please check if the following article helps you to understand the process. Though this upgrade was performed from version 12.2.8 the process is similar: