The difference between knowing Rails and knowing how to best apply its abilities come with experience IMO. While it provides an easy approach to web development it's still extremely easy to approach things in the wrong manner with Rails. There is a lot of bad Rails code out there written by people who've been doing using Rails for more than a couple of years.

Surely it would be useful with more than one year.

In addition to the core framework there are lots of modules to make your life easier (payment integration, multi-user system with admin dashboard, creating PDF invoices, HTML emails, etc.).

Presumably “experience” means having worked long enough with it to know how to quickly roll a solution for these things. Maintaining somebody’s custom CMS written in Rails 3 / Ruby 1.8 for a year would probably not provide you with such experience.

I've been at it since 2006 and I still learn things every day, fwiw.

It's kind of like asking if it takes very long to learn to use STL, or the standard C library. Sure, the libraries themselves are not that long if you're reading them straight through, but the capabilities they enable are pretty vast.