How short is the shortest fast?

Hey everybody, this is my first time here and first post!

I'm a vegan, aspiring to a 100% raw diet and have no addictions or bad habits like caffine etc.

On the advice of a friend, I recently cured myself of a head-cold in one day by water fasting. Rather impressed, I researched the benefits of fasting and I'm keep to go again. What is the minimum amount of time that it takes for Water Fasting to be beneficial? Obviously longer fasts yield better results, but are short fasts of 2-3 days damanging, simply unproductive or is there some worth to them?

Thanks in advance :)
- Tegan