It depends on the type of business you are launching. If it is a product-based business, then you are correct in that there will be significant runway/(and potentially investment required) to bring your product to market and start generating income. However, if you are a services-based business you should be able to start generating revenue much faster, if not at day 1. My past 2 companies were built on an agency layer so it was just a matter of pitching clients until we landed our first contracts. So I was fortunate in that in each case I got started by having someone hand me a check.
But like others have said, starting a new venture is going to be a balancing act. Assuming you are bootstrapping, ideally you will have saved up/set aside enough money to cover 12 months of living expenses. This removes the financial stress of not having any revenues/income, and you can be 100% committed to focusing on your venture.
However, the reality is that this isn't an option for most people, so having the security of a salary/benefits at your job (especially as you get older, have a family, mortgage, etc.) is a more realistic approach. This is fine, but then you need figure out how you can still be fully committed to launching your venture - i.e., will you put in another 8-12 hours a day (nights/weekends) *after* you get off of work? What about other responsibilities like time with your family? What happens as the business begins taking off, and the daily demands (during normal workday) increase?
No matter what, time is going to be your most valuable resource as there are only 24 hours in a day. Good luck!