If you want to use "forever," you can change the tense to present perfect progressive/continuous. "We've been going over this forever." Lawyers, college professors, and over-educated nerds like me might use "ad infinitum" in casual speech (strangely enough, I used it while talking with my parents yesterday). Most people wouldn't. I would guess that about half of Americans would not know the meaning of "ad infinitum."

You can't use "forever" as a synonym for ad infinitum here. You can say " we've gone over this a hundred/thousand/million times." You can say "we've gone over and over and over this." An educated speaker of English would understand and or use "ad infinitum" but "We've gone over this a hundred times" is more common.