GMAC's (the owner of the GMAT) philophy on implied text is as follows:

1) You can only imply what is already there.

• Correct: "I like apples as much as oranges" In this case, "I like oranges" is implied, and there is no problem because "I like" is present earlier in the sentence.

• Incorrect:"Last year I ate as many apples as the year before" In this 2nd case, what's implied is "as I had eaten the year before" (past perfect needed). Because this construct is not present earlier in the sentence, it cannot be implied

2) You can only imply if implying will not compromise sentence clarity. Basically this means that even if what you're implying is already there, the sentence will be considered incorrect if what is implied is far away or if the sentence structure is complex and re-stating the text is needed to keep the sentence clear. In the first example you posted, you are quite right to say that "for" is implied. However the sentence is complex enough that if an answer choice offers a more parallel option (specifically repeats "for") then it will have an edge. The rule of thumb is that if the implied text is close to the actual text, implication is ok. Otherwise, repeating the text is better.

As for the 2nd example you provided: "the manufacturer not only created monitor A but also monitor B". There is no way it can be a correct sentence. In the Official GMAT sentences that I've seen, what goes after "not only" should always be paired up (parallel structures) with what goes after "but also". I would think that there are two options to make this right:

• Put the text that will be implied outside the not only but also: "The manufacturer created not only monitor A but also monitor B"

• Actually restate the text; don't make any implication: "The manufacturer not only created monitor A but also created monitor B"

For similar applications. Look at OG12 SC #87 and at GMATprep question 2013

If you have trouble with parallelism, you can use the Drill Engine to generate timed drills and set topic='Parallelism, List AND Parallelism, Comparison' and difficulty='600-700 AND 700+'. As an alternative, I have developed flash lessons that test and explain how the GMAT deals with SC issues; one of these lessons is on Parallelism.

hope that helped,
-Patrick