How do I know when I need to go ahead and register an LLC?

To avoid wasting money, I would wait until you've made a few sales and you are certain you're going to pursue the business. But you might need a Tax ID to sign up for things like Stripe or Paypal or Bank Account, when signing up as a business. Sign up for all the services you will need asap so you know ahead of time what they require you to have. Also, look into registering in DE.

It really depends on your state. As @alexbecker, California is steep on annual payments, so you may want to hold off. I'm in Colorado and its only $50 to register and $10 a year, so its worth doing it very early. Otherwise, I would probably wait til you are signing up a customer.

If you live in California, keep in mind that California has an $800 annual tax on LLCs, regardless of which state you register the LLC and whether you make any money at all. So be sure you are getting at least $800 value out of registering an LLC before you do so--that's why I haven't yet.

As @theaccordance said,

1. When you want protection.
2. When you want to seperate personal from business income / expenses.
3. When you have partners.

I'd suggest setting up an SCorp as a parent company and then creating DBA's under the SCorp. This allows you theprotection of being a corporation, easy to do write-offs, you can still setup bank accounts for each project you start, it's easy to spin up or down a DBA but less easy to do that for an LLC. Lastly, you only have to deal with one EIN / Tax Filing at tax time. (Assuming of course you are in the US)

It looks like you are still in the idea phase, certainly no need to get an official company for the idea yet, it will just be a waste of time and money at this point. If you do freelance work and/or plan to make money eventually then do an SCorp and add stuff under it when it makes sense.

Generally speaking, it is possible to take a product to market as an individual; the resources are highly accessible depending on the product & regulation. I released my first Micro SaaS product without having an LLC.

That being said, my co-creators and I recently formalized our efforts under an LLC and there are clear advantages in doing so. You're able to separate personal liability, you gain access to resources (bank accounts), and you can separate revenues from your personal income, which can be advantageous when it comes to calculating taxes. YMMV depending on your country.

tldr: Check your local laws, if it's not blocking you, then don't let it get in your way of executing on an idea, and get it when it does make sense.