I did this a couple of years ago, and boy do I regret it. After many months of delayed, and new faces coming onto the team for a short period before leaving, there wasn't much hope to ever complete the project. I ended up accumulating debt (About 4.5 grand) that I am still paying off because I chased my dream.
Unfortunately, anything can happen when you choose to pursue a goal. It can get delayed, stopped, or outright fail. At the bare minimum, you would best be prepared to deal with delays, competing products, and outright failures. If you say "I have enough money to last me 12 months and I expect to take 7 months", then you best be prepared to answer:
- How quickly can you get a job if you need to?
- How do you prepare to deal with delays? What about delays caused by restructuring your time so that you don't go bankrupt?
- How do you prepare to deal with a competing product that is superior?
- How do you prepare to deal with corporate bullies who will try to steal or mimic your product?
- How do you know when the project has failed?
These are just a handful of ideas, and there are plenty more that would need to be addressed.
Probably the best thing that I have seen a few friends do is to ask for reduced hours. Working part time allows you more time while reducing, but not eliminating, the pay. Even better is that depending on your company, you could ask to go back to full time if your startup didn't work out.
Another option is to do what I'm doing currently: Find a job with lots of downtime. My job is critical and the market here is starved of good techs. Even then, I have a solid 2-4 hours of work each day. The other 4-6 hours I can spend on my personal projects that may eventually lead into a startup. If you plan to do this though, make sure to read your agreements carefully. There may be restrictions on copyright and the likes by working on a personal project on company property. If you do plan to go this route, you might want to consult a lawyer (like I did) to make sure you won't get screwed later.