Is 3 days effective?

This is the third time I've tried a 3-day fast (I'm on day 2). The first two attempts found me breaking the fast after just over 2 days. Then I read on this forum that some people are fasting for weeks, even a month. So my question: is 3-days enough to be effective?

I prepared for one week by cutting out meat & dairy, alcohol, caffeine and cigarettes. I still ate fish, noodles and rice and cooked veggies and raw fruits, and I felt a little weak with a headache after the first few days. I actually feel better now on day 2 of my fast than I did on day 3 of not injesting toxic stuff. I plan to stick it out this time until the morning after day 3. Is this enough to be effective? What is the ideal fast length?