You should test the upgrade procedure in your test environment and calculate the approximate time. I hope Meanwhile, you may get a good plan.
Please consider the below points before upgrade as well go through he below webcast to better understand the upgrade
run Gather stats program before upgrade
Workflow and Purging
Infrastructure, Database & Application Layer Tuning
Understand business functions and customizations
11.5.10 to 12.2 is quite a big jump - there are a lot of functional differences that you should be aware of that might affect any of your customizations. Things like AP invoice structures now have 3 level structure (invoice>lines>distributions) whereas in 11 it was only 2 levels. So if you've any custom code expecting the latter, you need to change it! There's too much to list in a post here (and in any case, I certainly don't know of every change, and I'd be amazed if any single person did!).