How to estimate how long it will take to automate tests?

I have been asked for metrics, figures etc on how long it takes to automate tests and how to evaluate the test automation effort.  I know, I know.  A piece of string is LONG.  How long???

I am more looking for documents that help estimate based on previous experience, complexity, preparedness.  GATNER reports etc would be great too.  I am interested in Functional Tests at the moment but have offered a performance example below.

I have a fair bit of automation experience and know it is hard to provide concrete estimates.  I am starting out from scratch with RFT and have not used it in the past so there is a learning curve.

When asked I provided an example from the past.  I attempted to automated 13 load / stress tests within 2 months.  Managed to get 11 running reliably, providing readable results / metrics and complete reuseability through data driven testing.  I know is sounds like a long time, but previously it had taken 3 guys a month to do the same tests using a different product.  This I felt was a starting bench mark, but not functional automation testing.