This discussion is irksome. First of all, why is it that we have become so apathetic to maintaining a standard? If we allow some things to slip by as "an evolving language," how do we decide which things to allow? This usage, "couple times," or "couple different," is a slippery little slope. I think allowing students, in particular, to use this...colloquialism/slang when writing formal papers is a massive mistake. Keeping it simple and devoid of existential discussion, it is a matter of simple mechanics. "Couple," according to Merriam Webster, is NOT an adjective, nor adverb nor a word of any other sort than this: it is a NOUN. Therefore, using it without "of," is, plainly, incorrect. For the sake of consistency, if you are a teacher, please do not let this "little thing" slip by. That is all.