upgrading Oracle forms and reports 6i to 12c We are planning a migration of an Oracle Forms and report 6i application to 12c, and at the same time we will upgrade to the latest version of Oracle (from 10g to 12c).

I would like to know the detailed steps that we can follow, or from where to start? 
Is there anything that we must consider before doing this? 
Any problems that we might encounter? 
How much time will it take? 
Taking in consideration that we have Over 900 .fmb/fmx files and Over 700 .rep/rdf files.

Any information whatsoever would be helpful !
I would recommend consulting with a consultant/company that specialises in migrating Oracle Forms and Reports. Why? Because we recently performed a similar migration and after 2 months we were not finished. 

Oracle has changed some things internally and features that previously worked can cease to function with the new version. 

Oracle has a special site dedicated to this task:

Fusion Middleware Upgrading Oracle Forms 6i to Oracle Forms 12c

I know this is not a complete answer, but so much depends on the complexity of your current environment, that it would be very hard to give you an estimation.