Estimatation for the testing process is a managerial task, sometimes also taken care by TestLeader.
two different techniques are mostly followed to estimate the testing process--1)Metrics-based and Expert based.
With the WorkBreakDown structure different stages,activities,and tasks are identified.We estimate regarding the effort needed ,dependencies,Resource Requirements.
Analyzing metrics,projects can be classified in terms of size(small,big,large) and the complexity can also be measured(simple,moderate,complex). Also from the past project activities estimation can be calculated.
Well not sure if this works for automation estimation or not but you can give it a try. There are various estimation techniques like Funtion Point Analysis. This can be used to estimate size of any software and based on that time can estimated. FPA is a method used to break systems down into smaller components so that they can be better understood and analyzed. Also it is technology-neutral so there should be any issue what technology you are using. I personally have used this technique to estimate the QA efforts.
Below is link which will give you more insight in to this technique. If you have any questions...feel free to get back.