Ask HN: How much of a SaaS needs to be built before fundraising?

I'm an aspiring founder and developer. I've worked on a few toy projects over the years, but have recently started on something that I believe has real potential.

I know next to nothing about the business side of things. I've been researching and learning, but its left me with questions. One such, at what stage in development should I begin fundraising?

Part of me feels like I should continue working on it during weekends and evenings until complete, and then start worrying about the "business" stuff. All the conventional wisdom however seems to argue against that and encourages swift market entry. How do I enter the market appropriately without funds? And how do I obtain funds without a fleshed out product or prior founding experience?

I have virtually 0 presence on professional social media outlets. No twitter, no IG, inactive linkedin, etc. My only viable options for marketing really is going to be paid ads. I'd also, ideally, like to be in a position to focus on this fulltime rather than worrying about it after my day job.

Now, it'd certainly be possible for me to just launch it one weekend and spend a few hour in the evenings managing things, but it'd seriously hamper growth and adoption.

Suggestion? Advice? Wisdom? I'm open to it all. Thanks in advance.