It really depends on you and your circumstances. But from my experience, I'd highly recommend being at least proficient in AutoCAD before depending on Civil 3D. We have had engineers join our team with no experience using AutoCAD and we expected them to be self-sufficient after a 3 day Civil 3D training course, it never worked.
You could probably learn to do some useful basic workflows, like produce profiles and cut and fill volumes. But to be able to produce drawings that are acceptable to issue requires a lot more skill and time using the software.
If your employer has a good template and some competent Civil 3D users you can seek help from then getting started in Civil 3D shouldn't be too difficult. If you are starting on your own then it will be very difficult without AutoCAD knowledge.
Mike Kingdon
Civil 3D Zealot
