A business isn't a business until you earned your first dime. So you definitely need to have some financial back up for your personal consumption which is separate from your business investment requirement.

Having said that there is no definitive timeline. thats why focus on earning your first business dollars, once you see that people are willing to pay for your service/product you can think of leaving the job.

For example if you have a foodtruck you will start money from day 1. Give it a go for a week or even a month in the evening after office before leaving your job. May be soft launch but try to understand if people are buying from you.

Often we make a mistake by providing free service/products at soft launch and people rush to get it. We see it as good sign and jump the bandwagon only to find out people are not willing to pay for it. So My two cents are see the dollar at first or have at least 12 months worth saving (consider at least one failed attempt) before jumping.