1) You can keep your job. Just always build things on the side. It keeps you coding for play and not just for work.
2) Try to sell the things you build. You don't need to be fully polished on day one. In fact, you shouldn't be and can't be because you need real customers to really understand their needs.
3) you can find the numbers online, but my saas project Cronitor had just $500mrr after seven months. You need patience and to adjust your work factor to match the available outputs. By letting it coast a bit while it picked up momentum we prevented burnout. When it started to grow faster we could pour some attention in and level up the product.
4) grow it while you work your day job. This is easy at first and grows harder. Having a partner is important here. Alternative: a business where a little downtime is not a big deal.
5) when it gets stressful, know your commitments. Your day job gets first bite and when you can't do that anymore you know it's time to move on and do it full time.
Most importantly the tldr is: quit your job after you've replaced most of your salary. And before you quit enjoy the incremental income.