note: This was initially a note for myself, but I think it could help others.
As soon as you have 18 months of runway (savings) ahead of you, you should leave your job and focus on your indie project fully. Doing otherwise will just increase the risk of your project never taking off.
When you just "have fun" on the side and don't care how long it takes, you will tend to do what's more comfortable. If you're an engineer, you'll spend your time refactoring the app three times. If you're a copywriter, you'll spend hours on a single sentence instead of putting out more content. If you're a designer, you'll overthink which font weight to use for every button type. And so on...
On the other hand when your life depends on becoming profitable, it forces you to go out of your comfort zone. You will find quicker and simpler ways to get to your goals. You will stay laser focused on the things that really matter. Even if you love to over-engineer stuff, you won't be able to. Your number one goal will be to make more money, not just enjoy yourself.
In 6 months you'll probably have a website, a few paying customers, and become clear on what you need to do to become profitable. If you keep your day job, in 6 months time you'll have finally figured out what color your "get started" button should be.
Take the plunge!
(Or prove me wrong 😄)