20 days because I don't know the details of the project, how complex it is, what is the nature of the project, how is the coding done, upgrading gems. It all depends. The time may increase if the existing one has complex structure. Or it may take less.
I have been working on ROR for quite a few years. I have worked on complex as well as easy stuffs in ROR.
Following steps I would be following for completing the project.
1. Go thoroughly through the project to understand it.
2. Whenever needed, I can contact you to clear my doubts.
3. Upgrading gems wherever required.
4. Changing the code to make it more readable.
5. Adding new functionalities if required.
6. Adding bootstrap for ui if needed.
7. Using Brakeman and RBP for error free code.
Do let me know if you are interested, we can go through details over skype.
Thanks