Have you talked to anyone about your product? Have you gotten any feedback about it? Do you believe it stands a reasonable chance of success if you devoted your full(er) attention to it? Getting feedback and product/market fit validation is relatively inexpensive. On the other hand, dropping everything to work on an unvalidated product can be quite costly.

I would recommend seeking out as much feedback as possible -- not just from friends & family, but from people on the street, people in your hypothetical target market, Show/Ask HN posts, meetups, etc. Look before you leap. While you can never fully mitigate the risk of starting your own business, you can certainly get your ducks in a row before jumping completely into it.

Maybe the product you want to build will never get built unless you devote 50%, 60%, or even 100% of your time to it. That's fair. But before you do that, prove the concept. Nothing is more heartbreaking that dropping a secure, comfortable job to leap into a project you haven't rigorously vetted and tested. Trust me; I've done that before. It didn't end well, and it set me back at least a couple of years.

One should never have to make a "cold turkey" move. Nothing about it should be cold. If you're giving up a good job to pursue something else, you owe it to yourself to make sure that something else might be viable. Instead of spending your 3-5% nights-and-weekends time building small chunks of the product, use that time to test the concept and the market.