You should eat whenever you are hungry! When you're eating a low-fat vegan diet, you need to eat plenty, in order to get enough calories. This is especially true for fruit-focused diets, because fruit is so low in calories.
Here is an easy-to-use website that quickly estimates your daily calorie needs. Be sure to select the option appropriate for you: "gain weight", "maintain weight", or "lose weight": http://www.calorieking.com/interacti...u-eat/?ref=nav
Then, just remember this calorie rule-of-thumb:
One cup of fresh fruit is 40-100 calories
One cup of boiled beans / lentils is about 230 calories
One cup of boiled grains / pasta is about 200 calories
One cup of non-starchy vegetables (kale, spinach, etc.) is 5-40 calories
One cup of nuts or seeds is 650-1000 calories
If you need 2000 calories per day (use the above calorie calculator to determine your actual calorie needs), you can see how much food you'll need to eat! If you are hungry, don't wait to eat! Allowing yourself to become hungry is not good, because one can become tempted to gobble down high-calorie junk food.
Mercy For Animals has a free, beautifully-written Vegetarian (actually vegan) Starter Guide. The nutrition part of the guide begins on page 7: http://www.mercyforanimals.org/files/VSG.pdf