There isn't any average downtime that anyone can provide.
A lot of factors govern the downtime - OS/hardware configuration, Customizations, topology of your environment to name a few.
Would advise you to review following:
Best Practices for Minimizing Oracle E-Business Suite Release 12.1.3 and 12.2.n Upgrade Downtime (Doc ID 1581549.1)
Oracle E-Business Suite Reducing Downtime Upgrading to 12.1/12.2 (Doc ID 1578187.1)
Hope this helps!!!