Actually, it is easy to quote a timescale with agile.
What isn't easy is saying you'll actually produce anything useful in that timescale.
That's the basic difference between Agile and Waterfall. Agile is time bound -- what can you finish in this amount of time? Waterfall is scope bound -- how long will it take you to do this?
Glen Ford, PMP
http://www.trainingnow.ca