Well I answered:
Estimation of automation depends upon the complexity of the process I want to automate, for eg an end to end test of trade (capture to settelment and finally reconciliation ) may take 10-15 days). Most of the time the estimates that I was asked for were verbal and I haven't created document for estimate. Generally, if I am clear about the manual process my estimates are pretty close, but that is not the case always; creation time exceeds when I have problems in accessing objects, or other such factors that need research"
As you can see my answer is not apt.
Is there any method for giving estimate of automation scripts creation time?
Well, you apparently gave a good answer, but to a different question. The question was related to script creation specifically. I would assume (and state my assumption) that the infrastructure was in place, and requirements documented. Then, depending on the number of scripts required (which would depend entirely on the nature of the process), I would estimate <some number of days> times the number of scripts.