I am not the kind of the developer that posts tons of updates, if at all. So take what I say with a grain of salt. But I think you should start when you know where the project is going, and you have a fair idea of how the final product is going to look and play, which is not as obvious as it may seem. I've started plenty of creative projects expecting to get something, and in the end get something completely different from what the original idea suggested me.