Absolutely agree with @SleepingPoppy , the right answer is going to be different for everybody- and whats right for you now can change! When I first started posting my comic in May, I had no buffer at all. I work full time and it took me a few months to get into rhythm where I was even able to reliably post an update every two weeks. It was ok when I was starting out, but it didn't last, I fell behind, and i had to take a small hiatus. Only after I started making my comic was I able to turn an analytical eye to my work flow and make the most of my free time. Now, I have a small buffer which is growing now that I've tweaked my habits to be more productive, but i wouldn't have been able to do that if I wasn't open to learning as I go and making mistakes.
Meet yourself where you're at now, and make what you can when you can. It'll work out - you'll be surprised at how patient people can be when you're patient with yourself. No magic number, just empathetic, steady progress!