In a spreadsheet break down the requirements (be they taken from a spec or from you analysing the AUT) and put a time to analyse, time to script and time to run against them.
Add them up.
You may have vague requirements and then would need to provide indicative times mentioned above for categories of 'easy', 'medium', 'complex' script. Then estimate the spread of each category of script your test will consist of.
Call it a best guess!
There are various estimation techniques available and can be used to measure the size of any software...these techniques are technologoy neutral. Basically this breaks down the software in to smaller components so that can be understood and analyzed better.
I think this can be used for automaiton as well..because automation is nothing but kind of development only. Below is the related link I found on web- http://www.spc.ca/resources/metrics/...estimation.pdf