If you really just start from a tutorial and get going you can get a serviceable project up - and understood - in about a week.
The development environment can be a little tricky starting out and may take a day or two. Or it could take an hour if you get the right tutorials. Some of the tutorials out there are a bit dated; anything mentioning Rails 3 will only be partially right.
Once you get the basics going you might want to read things like Eloquent Ruby to stop writing code that looks like converted PHP or Java.
Caveat - If you don’t understand MVC paradigm yet, figure twice the time or even longer.