What do you mean by "in detail"? Assuming that you first learnt C, how long did it take you to learn C++ "in detail"?
In addition RoR is two things, first there is the language Ruby, then there is the Rails framework which sits on top of Rails. How long it takes you to become proficient in Rails will depend on whether you have any web development experience as an understanding of html and probably javascript will be required in order to build web sites.
If you wish to proceed I suggest taking a couple of days out and working right through the tutorial at http://www.railstutorial.org, which is free to use online. That will give you an understanding of the basics of Ruby and Rails. You should then be able to answer the question yourself.
