to develop from scratch, a week or 2 being a noob(just backend no pretty interface). afterwards, any other site that is similiar, a day or so. once you get the hang of controller, model, view and learning to use/understand user documentation/api things start to get easy. ive been in the RoR environment for about 2 months now.
Maybe I'm just bit crap, but I do tend to think this underestimates the length of time it takes by quite some way, and is in danger of making us slowcoaches feel totally useless.
If you've got the app clearly thought through, understand the architecture and UI, and have a good understanding of not just Rails but Ruby too, and aren't thoroughly developing test case prior to coding, then these sort of timescales are possible.
However, there's probably a day or two -- at least, and that assumes no diversions, checking RSS feeds, other flow-reducing activities etc -- working out how to structure the app if there's any depth to it (though it won't necessarily be all at the beginning).
Add in a good dose of TDD, which will probably take 2-3 times as long as writing the code itself (quit looking at the browser), and you're talking weeks not days.
However, at the end, it will still have been an order of magnitude less than most other languages, and will probably be far more robust code with a stronger, more elegant structure (particularly if, like me, you come from PHP), and will have a good test suite that will make refactoring a cinch.
Just my $0.02