I've found a sort of sweet-spot where working a couple of hours before work, and sometimes on weekends forces me to really focus on what I'm building. So I don't see myself quitting my day job until the support workload becomes unmanageable.
For validating, I built a quick version in 7 days (https://onlineornot.com/building-saas-in-one-week-how-built-...), told the internet how I did it, kept building features, and customers kept coming back.
I built a dev tool that helps people jump-start React development projects (think boilerplate, but way more customizable). As soon as it hit $2k/month, where I could barely live off the revenue, I felt that was product market fit for me. It was solving a problem for some amount of people and I could continue to work on it full-time. Now doing ~$6k/month.
2. Get beta testers. Relevant comments on hn and reddit will get you first 10-20.
3. Set up a channel for live communication with beta testers. Discord works fine these days and is free. Listen to what they have to say. Be polite and grateful.
4. Ask beta testers "How disappointed would you be if this product ceased to exist?" on a scale of 1 to 10. This will measure how much they care.
5. If your starting scores are low (avg 5-6 or below) or you fail to make it to this stage, you probably do not have product market fit. If you have 8+, by all means keep doing it.
No, I did not stop my day job to start a new side project or company. The only time I quit my job was the very first time, I started the project. Took 2 years to close it with no big success. And I realized I should have done that without quitting my full time. Since then, I built 6 side projects, never tried to monetize. The latest one was started with an intention to make money, doing good since day 2. Concentrate on SEO if you believe the top 10 Google results of your focus keyword are doing bad. Once you get into top 2, thats all!!
My first launch was here on Hacker News, but at that time it was a prototype with no way to register and log in. Interest was very good. Then launched on Product Hunt where I got my first 20 registered users. Beyond that it has been posting the animations created on 4chan and other social websites.
I usually iterate on the features, look n feel etc. Try an idea, see if it sticks. If it doesnt, then discard it. I also have Discord channels that users can get help or chat on.
The product is an animation editor with no code or formulas https://toonclip.com
Then get people to sign-up to a mailing list for the MVP (or beta) announcement. The numbers help to verify your idea.
See mine, which will help people with questions like this: https://cxo.industries/
I would not advise you to quit your job. You can quit your job when your side project makes money equal to your salary. Set that as a bench mark and work towards that. Good luck!
Has YC addressed this in a meaningful way?
Startup School's biggest lesson has been "just launch and find out." You can wonder if you'll have product market fit forever, but you'll never know for sure until you have people paying for your product and telling their friends to sign up. It's by nature an ephemeral thing, startups are not yet a pure science.