It's not necessarily wrong. It's just a shortened version of "for a very long time". I can understand why you'd wonder though, since "for very long" sounds like you're talking about the measured length of an object.

It would be better this way: “I haven’t eaten this food for a very long TIME.”

"I haven't eaten this food for very long" is something you could say if "this food" means a type of food, you regularly eat this type of food, but not long ago you had never eaten this food. For example, if someone looks at the strange food you are eating and asks, "how does eating that food affect your health?", you could say "I don't know - until last month, I had never eaten this food before, so although now I eat it every day, basically, I haven't eaten this food for very long." But if what you mean is that you used to eat this food a long time ago, and then for a long time you did not eat it, but now you are about to eat it again (or you have just started eating), then you would say: "It has been a long time since I last ate this food." For example, if you ate a lot of spicy rice in 2016, 2017 and 2018, but then you stopped eating spicy rice and ate none at all in 2019 or 2020 or in 2021 until today, and then you sit down on Monday 20 Sept 2021 to eat spicy rice for the first time since 2018, you could say: "It has been a long time since I last ate this food." However if "this food" means, for example, today's dinner, then you could say "I have only just started eating this food" if you started eating your meal 5 minutes ago. If on the other hand you have finished the food and you ate it very quickly, you could say "I didn't spend long eating this food" or "it didn't take me long to eat this food".

for me.... it kinda sounds like you are in the process of eating it and you are saying, I am still eating..I have only just started my meal. If you mean you have recently been introduced to it, it is a new food to you.. you have only been eating it for the past few months....then I would say something like... "This food is new to me, I have only been eating it for 2 months."

Also: You couldn't say that in a positive statement. For example: "I have eaten this food for very long" would be wrong. You would say "I have eaten this food for a very long time".