upgrading Oracle forms and reports 6i to 12c
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.