How early is too early to start putting stuff on your wedding registry?

It's great to start gathering ideas, but keep in mind that some/many of the specific items you select will likely cycle in and out of stock/season multiple times, so that linens, appliance models, etc., you choose now may no longer be available when guests are ready to buy gifts in two years. Daughter put hers together about 6 months before her wedding and that was plenty early (and some items ended up unavailable and she had to keep readjusting).

I think making a general list right now and then maybe only adding things you know will always exist (a standard color Kitchen Aid mixer, for example). Like the 2 PPs said, things will go out of stock or be discontinued, so you don't want to get your heart set on something your guests won't be able to get for you. I think around the time you send Save the Dates is a typical time to have your registry go live.

You can start your registry at anytime. You can keep it private or just don't give it out to people until closer to your wedding date.