i doubt whether you will get industry benchmark for your query.
i suggest you arrive at your own benchmark from your previous projects.
in case you don't have any historical data use this figure "1.2 days" per test case for your current project. rebaseline this figure for your company after 2 to 3 projects and use that figure subsequently.
Yes you can use this figure for 12-15 steps each of 10-12 LOC.
Whether this estimate is rational or not depends on how much reusable asset you have and how much tailoring you have to do on the reusable asset to fit it in your project.
Some points I would like to clarify
1. The 1.2 is for those test cases which are data driven i.e this should cater for all test conditions. For ex: creating a order is a Test Case. All the Field level validations like "Date > Today" are test conditions.
2. Also it assumes a skill level of 5-6 out of 10 in WinRunner.
Adjust your estimate based on the above points and based on your reusable assets.
Once again you will be right person in deciding on your estimates. One advice though collect the data properly so that it can help you refine your estimates in your subsequent projects.