Vangaurd low cost index funds. Supposedly, you can count on 8-10% annual return. I also did/do some occasional stock picking which has gotten good results (but I wouldn't count on that being sustainable). I probably spend about 20k a year, and have savings in the low 6 digits.

I waited until I had 5-10x my annual expenses, but obviously you can do it with less. My main recommendation would to be have at least an extra 6 months of living expenses beyond what you need (so if you are taking a year off, have at least 1.5 yrs of living expenses) in case it takes a while to find work again. The main advantage of saving more than that is you can use the returns from your investments to offset some of your costs a bit.