I don’t think it’s really ever too early to start. The thing I’ve found is that you need to focus on the ‘business’ your starting and not entirely on the product. If all your talking about is a product that’s 6 months away, that’s boring. If, however, you talk about the process you’re going through and talk about the product in terms of the process then that’s interesting. You’ll be able to build your reader base that way and it’s great to have a reader base before the product is done because your readers can give you some great ideas. I know mine have.