I'm a firm believer in learning by doing so I'd say you should begin as early as possible.

In those stages, knowing how to search is important. Try to understand the problems you deal with and look for features and/or tools that solve them. Get to know the documentation and surf through Q&A sites.

That's how I've been learning since I started in college and it's served me well throughout years; move fast and break things, as they say. Just be careful not to release/publish anything broken.

“If I had eight hours to chop down a tree, I’d spend the first six of them sharpening my axe.”

-Abraham Lincoln

I don't think there's any set point in time when you go from formulating an idea to moving on to the actual development of it. If the idea is strictly for learning web technologies, then I would say jump in as soon as you can, assuming you're one of those people who learn by doing. If you tend to learn more by reading articles/books/blogs and watching videos, then go that route.

The only time when timing could be a potential factor is when the web site is used for practical purposes, the longer you wait to start coding it, the more likely someone else may come along and get it out to market before you.