Inception can take many forms. For some projects, it's a chat at the coffee machine: "Have a look at putting our catalog of services on the Web." For bigger projects, it might be a full-fledged feasibility study that takes months.
During the inception phase, you work out the business case for the project ” roughly how much it will cost and how much it will bring in. You will also need to get a sense of the project's scope. You may need to do some initial analysis to get a sense of the size of the project.
I don't tend to make a big deal of inception. Inception should be a few days' work to consider whether it is worth doing a few months' worth of deeper investigation during elaboration (see the next section). At this point, the project's sponsor agrees to no more than a serious look at the project.