I'd agree that at least a year's worth of expenses in savings is needed. You also have to make a realistic estimate of how long it will take you to either start making a profit or obtain other funding for your business. If you're doing something where people typically are only breaking even after a year, you need to take that into consideration and have a bigger nest egg.

12-18 months is a good number. It allows you enough "run-way" to get something launched and possibly funded

I agree with Ezra, error on the high side and consider those cash needs that might come up in the next 1-2 years, such as car replacement or home improvement needs.