This varies widely depending the type of course, the course design, your experience level, and your technological skills. It also depends on whether you are creating a new course from scratch or are converting an existing face-to-face course to online delivery.
For a new fully online course, plan on a minimum of 120-180 hours of development time. For a hybrid course that replaces half of the "seat time" with "web time," plan on 60-90 hours of development time. For a web-enhanced course, plan on 1-2 hours per document that you plan to put online, assuming the documents are already written. Allow double that amount of time per quiz or exam. If your course includes many media elements (graphics, video clips, audio clips, animations, etc.), the development time will increase. Remember to allow time to attend training that covers instructional and technical topics.
You also need to plan ahead. The e-Learning Center recommends that faculty allow at least six months of development time for creating a fully online course. It is not 6 months of full-time work on the course but it allows faculty members to fit in web course development among their usual teaching loads and other commitments.