Could you perhaps fill in your language profile?

Must admit, I have no clue what rules apply but from the "feeling" I'd say:
Our body tries to guard itself for as long as it could
speaking of a body, I would rather use "guard"

The "for" is optional, but adding/removing it changes the meaning of the sentence.

"As long as it can" would mean something like "solange er kann", while "for as long as it can" would be more like "möglichst lange."

"Our body tries to guard itself for as long as it could" aus der #2 scheint mir aber nicht richtig, oder?

#4 "Guard" and "could" both look strange to me. I would stick with the original sentence.

If adding or removing a word from a sentence changes its meaning, it's not optional. In the OP's sentence, the "for" is indeed optional because the sentence retains the same meaning with or without it. On the other hand, I don't think it's optional in most cases (and certainly not in formal sentences), and "for," in this type of context at any rate, emphasizes a length of time and is needed.

If adding a word changes the meaning of the sentence, and the meanngs of the sentence with and without the word are both acceptable, then the word is optional.

If omitting/including a word changes the meaning of a sentence, that meaning has obviously changed, i.e., is not the same. How can such a word then be "optional"?

If both meanings are acceptable, you are free to use the word or not use it. It is optional.

I have to agree; I do not believe for changes the meaning of the sentence, #3 notwithstanding.

But it is not needed, so the sentence is better without it.

I think you're comparing apples with oranges, RTH01. But I also disagree with Bob C. that the sentence is "better without it." I think it sounds more formal with "for" and more colloquial without it.

Of course everyone is free to pursue their own style, even if that means dropping in redundant words.

As long as you think that's what's happening, Bob ...

The two versions (with and without "for") aren't interchageable. They have different meanings and involve different grammatical constructions.

In the first, without "for", "as long as" is a conjuction, meaning "provided" (as in "you can come as long as you leave after I do").

The second uses "for", indicating a length of time, plus "as ... as ..." (as in "you can come for as long as you like"). This is presumably the form intended in #0.

The reason these are getting confused is that there are sentences such as "I'll stay/live/survive (for) as long as I can", which keep the same meaning if the "for" is omitted. But these are special cases. The verbs involved can take a length of time as a direct object. That doesn't work with "protect" though. "Our body tries to protect itself three hours" is poor English, and so is "our body tries to protect itself as long as it can", unless "as long as" is being used as a conjunction.

I would use the for.
I don't think that omitting it changes the meaning.
I had to reread #14 twice to understand the point being made. But perhaps it does point a possible ambiguity, which would support the use of for for clarity.

In the sentence

Our body tries to protect itself as long as it can.

as long as can mean either if it is able to do so or for such a length of time as it is able to do so.

Without context, it is not possible to say which. However, our body tries to protect itself as long as it can meaning for such a duration as is possible is not poor English but completely grammatical and idiomatic.

OK. I'm not going to argue with that.

Maybe it's a regional thing, maybe it isn't.